Abstract-The present study tried to assess the roles of vocabulary knowledge in reading comprehension of Iranian EFL learners. Using the multivariate analysis, this study examined the roles of depth and breadth of vocabulary knowledge in reading comprehension of a group of Iranian EFL University students with a minimum vocabulary size of 3,000 word families as was measured by Schmitt's (2001) Vocabulary Levels Test. The study found that 1) the test scores on vocabulary breadth, vocabulary depth and reading comprehension are positively correlated and 2) vocabulary breadth was a stronger predictor of reading comprehension than depth of vocabulary knowledge for the participants of the present study.Index Terms-breadth of vocabulary knowledge, depth of vocabulary knowledge, lexical threshold for reading comprehension