The present study investigates the mediating role of language aptitude and the effect of metacognitive vocabulary learning strategy instruction on the recall of collocations. Therefore, 75 high and low language aptitude level participants were randomly assigned to two control and experimental groups based on the TOEFL test score at upper intermediate level and the Words in Sentence component of the Modern Language Aptitude Test-Elementary (MLAT-E) (2002). Both experimental and control groups received the same type of collocation instruction, but the experimental group, in addition, received the metacognitive explicit strategy instruction. A pretest and posttest measuring the learners' collocation knowledge before and after treatment was administered. During the training, the class time was allocated to teaching collocations and the last thirty minutes of each session was dedicated to metacognitive strategy instruction in the experimental group. Treatment continued for eight weeks and the required data were obtained. At the end, a two-way ANOVA was run to compare the two groups plus the effect of language aptitude on such performance. The results indicated that treatment did have an effect on the recall of collocations and also the high language aptitude learners enjoyed better performance compared to their low language aptitude counterparts.