Based on the consensus that vocabulary plays an essential role in improving reading ability, the present study investigates the effects of two different methods of English vocabulary instruction - the traditional wordlist-based vocabulary instruction (G1) and the collocation-based vocabulary instruction (G2) - on improving English reading ability for high school learners. First, the results of the collocation test revealed that G1 and G2 groups attained similar mean scores on the pretests; however, G2 significantly outperformed G1 on the post-test administered at the end of the learning activity. The results proved that students who learned vocabulary in a collocation-oriented setting had better retention through the lexical-chunking achieved by the collection method. Second, in terms of reading comprehension achieved on nationwide academic trial tests, the result was, after considering more tests, that G2 gained a significantly higher mean score on the third and fourth than G1 did, although the two groups yielded a similar mean score on the first and second test. The implication of these results is that collocation-based vocabulary activities had a more significant influence on reading skills than wordlist-oriented ones.