This study investigated the effects of reading graded readers on the reading comprehension scores of the prep students at university. 30 EFL students from School of Foreign Languages (SOFL) in Selcuk University participated in the study. Two groups, control and experimental group, each consisting of 15 students were formed at the beginning of the study. Both groups attended traditional language classes of SOFL, and the experimental group was additionally involved in an extensive reading program as an extra-curricular activity. The program was performed in the second semester of the academic year. Reading comprehension development of the groups was assessed by means of reading comprehension tests given as part of the third and fourth mid-term exams set by SOFL. Findings indicated that students in the experimental group scored significantly higher than those in the control group at the end of the extensive reading program. Also, the amount of reading had a significant effect on the reading comprehension scores of the students; students who read more books achieved more reading comprehension development in comparison to those who read less. Consequently, results of the study revealed that extensive reading had a positive effect on the reading development of EFL students.