This study attempted to investigate the relative contribution of vocabulary and grammatical knowledge to second language reading comprehension ability. With 44 EFL adult learners whose language proficiency fell within the low-intermediate level, three different measures were conducted: vocabulary and grammatical knowledge were measured by researcher-developed tests and L2 reading comprehension was measured using the reading section of the TOEIC test. In order to examine the two predicting variables (i.e., vocabulary and grammatical knowledge) a multiple regression analysis was performed. The study found that both grammatical and vocabulary knowledge related significantly to L2 reading comprehension. Additionally, vocabulary knowledge showed a slightly stronger contribution to reading comprehension than syntactic knowledge. These findings suggest that it is equally important for lower level readers to understand vocabulary as well as basic syntactic rules and structures when it comes to EFL reading comprehension.