Objectives In this study, for the purpose of improving 26 4th graders’ English learning interest, a homeroom teacher conducted an action research study and helped their mobile-app-based English learning and her adviser supervised the teacher to conduct the research study successfully, according to qualitative research tradition. Although many theorists and scholars propose promising effects of mobile-app-based English learning there is not much mobile-app-based English learning research conducted with primary school students and teachers at public primary school settings. This is an action research study to fill the gap in the previous studies.
 Methods During the spring semester of 2022, 26 elementary school students in a southern province performed mobile-app-based English learning activities with their homeroom teacher in a 30-minute morning-class session and in a 40-minute club activity session. Teacher observation reports, survey results and interview data are collected before, in or after the mobile-app-based English teaching/learning class.
 Results The first result is that almost all of the 26 students liked the English learning activity of singing English songs using YouTube app. As for the second result, among the four English learning apps applied in the study, most of the students liked the Dino Buddies series of the Little Fox app and the Kahoot word quiz app the most, and low-English-learning-autonomy students liked the ‘FunEasyLearn’ English app the most. The third result is that all 26 students' interest in learning English improved while participating in the study, with varying degrees of improvement. The fourth result is that the preference for mobile-app-based English learning activities varies greatly by student according to the level of English-learning interest improvement.
 Conclusions The results of the study showed that 26 students who conducted English learning using mobile apps for one semester developed interests in learning English and that they all had various preferences for mobile-app-based English learning activities. Based on the study results, it can be predicted that the learning effect will be maximized if mobile-app-based English teaching is carefully planned and implemented according to the characteristics, autonomy, personality, and English proficiency levle of each student as a learner. In particular, when carrying out mobile-app-based English teaching for students with low autonomy, it would be very helpful if an individual learning helper or an assistant teacher helps their learning. It seems that further research is still needed to know how to implement mobile-app-based primary English education for Korean learners.