The present study aimed at investigating the effect of two vocabulary learning strategies on promoting mentally retarded EFL learners' vocabulary learning in short-term and long-term.To this end, 32 mentally retarded EFL learners (with IQ levels of 50-69) from a mentally retarded institute in Sanandaj, Iran were selected as the participants of the study. First, some English words were selected. Then, the first group was taught through flash cards on which the pictures of the words were depicted. The other group was taught through translation into their L 1 which was Kurdish. After the treatments, which took about five weeks, the participants took a vocabulary post-test. Two weeks later another vocabulary post-test was administered to the participants to investigate the long-term effect of the vocabulary learning strategies. The obtained data were analysed through running two independent samples t-test analyses. The results showed that there was a significant difference between the effectiveness of the two vocabulary learning strategies. The group that learned vocabulary via pictures outperformed the other group in both post-tests.