This research aimed to study a direct positive effect of English self-efficacy on English language proficiency, a positive effect of metacognitive learning strategies on English self-efficacy, and an indirect effect of metacognitive learning strategies on English language proficiency with English self-efficacy as a mediator among 160 EFL undergraduate science students. Three research instruments were employed to collect data, namely, a questionnaire of English self-efficacy, a questionnaire of English self-regulated learning strategies, and an English exit exam. The data were analyzed by means of the structural equation model (SEM) with a second-order confirmatory factor analysis (CFA) using LISREL software. The findings confirmed a direct effect of English self-efficacy on English language proficiency, a positive effect of metacognitive learning strategies on English self-efficacy, and an indirect effect of metacognitive learning strategies on English language proficiency with English self-efficacy playing a mediator role among the participants.