This study investigated the effects of a self-developed mobile app on Chinese university EFL learners' vocabulary learning and retention. Their perceptions of the mobile app were also explored. Data were collected using mixed methods. The results can be summarized as follow: firstly, the students using the mobile app could learn more words than those using the paper-based word list; secondly, the students in the experimental group could retain more words than those in the control group; thirdly, based on students' responses on both the questionnaire and the interview, most of them perceived the mobile app to be convenient and easy to use for vocabulary learning. Additionally, most of them believed that the mobile app was very useful and effective for their vocabulary learning and retention. Finally, all of them enjoyed using the mobile app to learn and remember EFL vocabulary.