This study investigated how a digital game-based learning (DGBL) environment affected low- (LP) and high-proficiency (HP) primary school students' self-regulated learning strategies and vocabulary development. Data were collected from 62 sixth graders (20 LP and 20 HP students) who learned English as a Foreign Language (EFL). The statistics suggested DGBL benefited both LP and HP students' self-regulated learning strategies and vocabulary development. DGBL encouraged LP students to develop cognitive, metacognitive, and resource management aspects of self-regulated learning strategies. With self-regulated learning strategies, LP students could benefit more from DGBL than HP students did in vocabulary development, reducing the gaps between LP and HP students. The prediction of the simple regression analysis also showed self-regulated learning strategies in DGBL contributed to students' progress in vocabulary learning.