The purpose of this research is to explain the effectiveness of using online mobile games to enrich students’ vocabulary acquisition in teaching the writing of the narrative text. This study employed quantitative method. The research design was Pre-experimental research, One-Group Pretest-Posttest Design. The research participants were the 8th grade students at SMPIT Cahaya Ummat, Semarang regency. Data collection technique used pre-test and post-test. Data analysis technique applied data reduction, data display, and data validation. The results of the study show that online mobile games being an effective media for students in teaching writing narrative texts. The researcher concluded that online mobile games can be one of the effective media for students to enrich vocabulary acquisition in teaching writing narrative text.