Solar collectors should be oriented with the right tilt angles in order to get the maximum solar radiation in the application of solar energy systems. Thus more solar irradiation falls onto the solar collector by a straight angle. The right tilt angle varies depending on time and the geographical position of the specified zone. To minimize the effect of these parameters, solar collectors should be oriented with the optimum tilt angle for the corresponding zone and time period. In this study, optimum tilt angles are determined for four cities in Turkey at different degree-day regions in order to obtain maximum performance from the solar collector. For this purpose, the cities İstanbul, Ankara, Erzurum, Adana are selected as representative samples for four different degree-day regions in Turkey. Optimum tilt angle for each city and each month during the year is calculated separately using theoretical calculation and Hottel & Woertz methods and the results from both methods are compared with each other. According to the results of theoretical calculation method, the maximum optimum tilt angle is obtained as 56° for Erzurum in December, while the minimum angle is obtained as 12° for Adana in June. The maximum difference between the optimum tilt angles calculated by the two different methods is obtained as 19.3% for İstanbul in January, while the minimum difference is obtained as 4% for both Ankara and Erzurum in July. Mean difference between two methods is 12.64% when taken all cities and months into consideration.