The aims of the six sigma philosophy which doesn’t accept the idea of the defects are inevitable in the processes are to produce defects free at he first time and to reach to the goal of 3,4 defects per million opportunities by minimizing the variation and meeting the customers’ expectations. Six sigma has been firstly applied in Motorolo in 1980s and then it has been widely used by other manufacturing firms around the world. By six sigma applications, the manufacturing companies had many opportunities to increase product quality, productivity of the processes and to decrease costs. By the applications of six sigma in the organizations, the customers’ satisfaction can be increased as well as the organization can gain many financial benefits. According to Pyzdek (2003;5) the firms which are at 3 or 4 sigma level spend the 25-40 percentage of their revenue for fixing the problems. The firms which are at the six sigma level spend less than 5 percentage of their revenue for fixing the problems. By the 2000s’ the service firms also started to apply six sigma in their processes. Particularly, healthcare and financial companies had many successful six sigma applications. Six sigma can be applied in service firms successfully however service firms can meet with many difficulties related with the characteristics of the service firms. Depending on these difficulties the service firms should evaluate the critical success factors of six sigma applications in service processes. In this paper, depending on the literature review the success factors for six sigma applications in service firms have been evaluated according to the differences of the manufacturing firms and service firms in the application of six sigma and the difficulties that the service firms meet while applying six sigma projects. Evaluating the success factors for six sigma applications in service firms is important for increasing the efficiency of six sigma applications in service firms. According to literature, many service firms operate at the 1,5-3 sigma level. So, six sigma methodology present many opportunities for service firms to decrease their defects and to increase six sigma level. In the literature there are many success stories about the application of six sigma methodology. These examples are mainly from healthcare and financial sector. In addition, there are some six sigma application examples in the service departments of manufacturing firms such as human resources department, in education and in the activities of municipality. Evaluating service quality is more difficult than evaluating the product quality and service quality is related with the differences between the perceptions of the customers about the service and the expectations of the customers from the service. The main characteristic of service firms is variability. Due to variability in service processes , there can be many difficulties in the applications of six sigma in service firms. Service firms may meet with many difficulties in the measure phase of the six sigma methodology. In many applications, the data is not ready for the analysis and the characteristics of the data may cause not to able to use statistical techniques extensively. In service firms, which data will be gathered, how data will be gathered and which analyses will be done are very important steps in six sigma applications in service firms. If these steps can be done successfully one of the challenges in the application of six sigma in service firms can be eliminated. According to the literature one of the difficulties in the application of six sigma in service firms is about defining the process parameters. In addition to these difficulties, to define and to quantity the defects are not easy in service processes and generally it is related with the personel. The defects in six sigma applications in service firms are related with customers’ satisfaction and the examples of the defects in service firms may be long lead time, inadequate quality and high cost. In order to determine these problems it is necessary to have a picture of the service processes to specify how these process are good to meet the customers’ expectations. According to literature, for the success of the six sigma applications, the support of the management is very important. In the applications of six sigma in service firms, the management must support six sigma applications by investing for six sigma applications, by separating resources for six sigma applications and by providing the involvement of all of the personnel in the organization. Cultural exchange must be provided for six sigma applications. However this cultural exchange for six sigma applications may be more difficult in service firms than manufacturing firms. According to literature use of the flow diagrams and process maps are not used commonly in service firms. In addition the use of experimental design in the six sigma application in service firms is very limited. 5S, visual control, value stream mapping can be applied in service firms successfully. SIPOC, project charter and critical index matrix have got important roles for the success of the six sigma projects. The statistical techniques can’t be used so commonly due to the characteristics of data in service firms. Generally the data is not normally distributed and not in process is no control. Because of this factor, nonparametric techniques can be used for statistically analyses in six sigma applications. Service firms meet with many difficulties in the application of six sigma. However this doesn’t mean that six sigma can’t be successful in service firms. There are many service firms which decrease their cost and improve the efficiency of their processes and quality of their products. The application of six sigma in service firms may be more difficult than the applications in manufacturing firms because o