The current study is an attempt to investigate the particular role learners' vocabulary knowledge plays in their reading comprehension performance. It intends to determine whether breadth and depth of vocabulary knowledge are related to EFL learners' reading comprehension, and to investigate which one of these variables, that is, depth or breadth of vocabulary knowledge, makes a more important contribution to L2 reading comprehension. It also attempts to investigate whether there is a relationship between these two vocabulary knowledge dimensions, that is, depth and breadth. Finally, the study tries to find out whether gender has any effect on learners' reading comprehension and vocabulary knowledge. The participants of the study were sixty (30 male and 30 female) EFL learners who were chosen from among five language teaching institutes in Shiraz based on available sampling. To collect the relevant data, two tests measuring breadth and depth of vocabulary knowledge were administered to all participants. They also received a reading comprehension test in which they were asked to read the passages and answer some multiple choice questions. The results obtained from the analysis of the data indicated that while both depth and breadth of vocabulary knowledge play an important role in EFL learners' reading comprehension performance, depth of vocabulary knowledge makes a more important contribution. The results further revealed that depth and breadth of vocabulary knowledge are positively correlated, that is, those learners who had large vocabulary size had a deeper knowledge of the words, too. It was also found that gender had no significant impact on learners' reading comprehension performance and vocabulary knowledge.