Although increasing evidence has demonstrated the benefits of mobile technology in diverse aspects of language learning, research on the use of mobiles in task-based instruction has been scarce. Particularly, there has been little research directly investigating predictors of the quality of mobile-assisted, video-making tasks. To fill the gap and contribute to the area of emerging technology-mediated, task-based language teaching, the current study examined predicting factors of the quality of digital-video-making task outcomes. Forty-eight Korean high school students completed a digital-video-making task using a mobile application called KineMaster. Students created a self-introduction storytelling video focusing on their experiences of success or failure in their life. To examine predicting factors (English writing proficiency, first language narrative ability, digital literacy, intrinsic and extrinsic motivation), multiple data sources were gathered: timed argumentative English essays and Korean writing samples on the same narrative topic, a digital literacy questionnaire, and a motivation survey. For data analyses, the quality of the digital-video-making task outcome was scored using an analytic rubric assessing the use of multi-modes, language use, and task fulfillment. Students' English and Korean narrative essays were rated for English writing proficiency and first language narrative ability, respectively. Digital literacy and motivation questionnaires were analysed quantitatively, and the composite scores were included in statistical models. Multiple regression models were created to investigate which factors predicted the quality of the digital-video-making task outcome. The results showed that English writing proficiency and first language narrative ability significantly predicted the quality of language use and task fulfillment. The findings shed light on mobile-assisted task performance in foreign language classrooms.