There are two main purposes of this study. The first is to determine the basic difficulties of students in solving problems about the theory of special relativity, and the second is to identify the possible causes leading to these difficulties. To achieve these goals, 3 open-ended questions were administered to the participants. These questions were answered by 78 students who had been enrolled in a modern physics course. Out of 78 students, 24 students were selected for interviewing. Commonly used textbooks were also analyzed in order to determine whether there are differences in content. The results of the analysis showed that the students had difficulties in selecting the proper frames of reference. In addition, they could not use the concepts included in the fundamental equations accurately in accordance with their meanings. The main reasons behind these difficulties seem to be the improper generalizations of students. Moreover, the different representations of the fundamental equations in the various textbooks were also seen as the reason for the difficulties students encountered.