This study examined the effect of using a digital game-based language learning mobile application "Liulishuo" (speaking English fluently) to develop complexity, accuracy, and fluency of English monologic oral production among 30 English language learners in China. Monologic oral production was measured using the same narrative picture description task in pre- and post-tests. The learners followed the "Imitation of English Monologues" game 30 minutes each time, twice a week, for 20 weeks. The oral production was measured using six indices: the mean words per T-unit and lexical density (i.e., complexity), the mean repairs and errors per 100 words (i.e., accuracy), speech rate and the mean length of pauses (i.e., fluency). The paired sample t-tests showed that the participants produced more complex monologic speech, had significantly fewer errors, and increased speech rate, but the mean repairs and mean length of pauses remained unchanged. The unchanged repairs and pauses could be possibly due to the non-proceduralized linguistic knowledge in oral production, which may require a more extended period of treatment. Our study showed positive effects of using a digital game-based language learning mobile application on the improvement of complexity, accuracy, and fluency of English language learners in China' monologic oral production with varying effects.