The present study examines L2 reading proficiency effects on the relative contribution of vocabulary knowledge and grammar knowledge to L2 reading comprehension for Korean high school EFL learners. To this end, 200 high school students were asked to take a vocabulary knowledge test, a grammar test, and a reading comprehension test. The participants were divided into three sub-groups by L2 reading ability in order to examine L2 proficiency effects. Multiple regression analyses for the sub-groups indicated the relationships among the three variables as distinctive. The results showed that syntactic knowledge had a predictive power for reading performance in the high reading group, but vocabulary had the same quality in the intermediate reading group. For the low reading group, neither vocabulary nor grammar could significantly account for the L2 reading variance. Theoretical implications and directions for further studies are discussed.