It is an essential subject to examine the thought of the letter and science of the letter in the work (Rasâil Ikhwân al-Safâ) of Ikhwân al- Safâ, which is a secret society of Muslim philosophers and which came to exist in 10th century in Basra. The science of the letter, whose history goes back to ancient period, has three essential aspects. These are summarized by these means: 1. To understand the mystery of beings by the letters, to attribute meanings to the letters, to correlate connections between the letters and the numbers, to explore the mysteries and to have a grasp of aspects of secrets of beings by the various laws of ratio and harmony. 2. By the recognition the effects of the letters on the beings, to provide this effect by the way of amulet, talisman, cryptogram (wafq), magic etc. 3. To reach invisible and futuristic knowledge by the letters. In this study, whether Ikhwân al-Safâ provides these three sides or not will be discussed. At this point I have tried to present their ideas on letters in Resâil and I have reached a result whether they are Hurufist or not. As a result, our researches have illustrated that Ikhwan al-Safa interested in the science of the letters and made evaluations on some dimensions of this science, but they were not interest in the third aspect too much. Moreover they were interested in the second aspect partially. Their approach related to second and third aspect is mostly descriptive and in explanatory situation. Therefore it is difficult to see or accept Ihwan al-Safa as a heated defender of the science of the letters and complete advocator of Hurufism. It will be more correct to evaluate Ikhwan al-Safa as group who made a sort of mysticism and symbolism of the letters and working on the science of the letters a certain extent.