This research examines the effects of the explicit instruction of English loanwords in vocabulary learning on 74 junior college students who were divided into two groups (a treatment group and a contrast group) according to their English proficiency. Explicit instruction of loanwords along with regular vocabulary instruction was provided to learners in the experimental group in the L1 (Japanese) while the control group was provided with only regular vocabulary instruction. This study investigates whether explicit instruction in the L1 enhances the learners’ lexical understanding of loanwords as well as their receptive vocabulary. Two explicit instruction sessions were given during regular class periods. The results of a vocabulary post-test given one week later and a delayed test eight weeks later indicated that the treatment group outperformed the control group on the post-test but showed a decline in mean score on the delayed post-test and that both English learners of higher and lower proficiency levels in the treatment group produced great learning gains on the post-test but showed no statistically significant difference detected eight weeks after learning. The initial conclusions of this study suggest that explicit instruction in the L1 of English loanwords suggests a positive correlation in the retention of lexical knowledge with relatively short period of time despite learners’ English proficiency.