This article reports on a study exploring primary students’ after-class social interactions in English vocabulary acquisition using a mobile learner-generated-content (m-LGC) tool. A total number of 29 grade 4 students from an elementary school in Hong Kong were involved. A case study approach was adopted. Data collection included log data on the m-LGC tool and semi-structured interviews. Data analysis included content analysis, visualisation using Gephi, and thematic analysis. The results show two types of students’ social interactions in a m-LGC learning environment. The interview results indicate that students held different perceptions of social interactions using the m-LGC tool. The implications are discussed. Copyright © 2022 Yin Yang, Yanjie Song.