This study was conducted to investigate the extent to which scores on depth and breadth of vocabulary knowledge as two dimensions of vocabulary knowledge would contribute to predicting the EFL learners' reading performance with a minimum vocabulary size of 3000 word families and also to find out the difference, if any, between the reading comprehension performance of two groups as having high and low depth and breadth of vocabulary knowledge. All instruments (Word-Associate Test (WAT) (Read, 1993) measuring depth of vocabulary knowledge (DVK), Vocabulary Levels Test (VLT) (Nation, 1983) measuring Vocabulary Size (VS), and Reading Comprehension Test (RC) were administered together within the same session to 38 Iranian senior university students. Then the gathered data were analyzed by a Pearson correlation and two independent t-tests. Results revealed that (a) the interrelations among DVK, VS, and RC were high and positive; (b) depth of vocabulary knowledge provided a significant contribution to the prediction of reading comprehension; and (c) the students who had stronger depth and breadth of vocabulary knowledge performed better on reading comprehension. These findings provided empirical support for the importance of depth of vocabulary knowledge in reading comprehension. Regarding this, the necessity of incorporating this aspect into EFL programs and activities is desirable. (Contains 7 tables.)