Abstract: The present study attempts to shed light on the role of depth of vocabulary knowledge in reading comprehension and its relationship with grammatical knowledge. To this end, a pre-1995 TOEFL and a measure of depth of vocabulary knowledge was administered to 57 EFL learners (17 male and 40 female). The analysis of the results showed that (a) language proficiency influences performance on depth of vocabulary knowledge tests (b) although depth of vocabulary knowledge is significantly related to reading, grammatical knowledge explains the greatest amount of variance in tests takers` performance on reading comprehension tests and (c) knowledge of collocation is related to grammatical knowledge. Having presented the findings in detail, the present study advises language teachers to increase the grammatical knowledge of language learners through diverse means such as focus on form and explicit grammar instruction.