Abstract: The purpose of this study was to investigate the contribution of different aspects of lexical knowledge to reading performance of the students. To answer this question, a test of vocabulary, which was adapted from Zareva (2005), was administered to 157 Iranian B.A. university students majoring in English literature. The study also used the students' GPA in their reading courses. It was shown through regression that among different aspects of vocabulary knowledge studied in this study only vocabulary size and self-perceived vocabulary knowledge contributed to students’ reading comprehension. It was also revealed that breadth, and receptive productive dimensions of vocabulary knowledge, accounted for high amount of the variability of students’ reading scores. Moreover, considering that part of data collected from freshman students, it was demonstrated that Depth of vocabulary, had the highest contribution to reading after Breadth.