This study investigated the relationship between teaching L2 semantic tasks (collocation vs. definition) in vocabulary achievement of Iranian intermediate English as a Foreign Language (EFL) learners. To this end, 60 students at intermediate level studying in the Simin Institute were selected from a total number of 100 participants based on their performance on Oxford Placement Test (OPT). After ensuring the criterion of homogeneity, they were randomly assigned to two experimental and one control group each comprising 20 participants. Before starting the treatment, a pretest was administered to find out learners’ vocabulary knowledge. Then the experimental groups were instructed to use collocation and definition in vocabulary learning for 5 sessions while the control group received no treatment. After the end of the treatment, the same pre-test was administered to all groups as a post-test. Analysis of the results of three paired sample T-test and a one way ANOVA showed that the experimental group that received collocation task outperformed the other two groups (control group and second experimental group) in the post-test. It was concluded that teaching collocation task had a positive effect on learners’ vocabulary knowledge.
