This qualitative case study examined a group of undergraduate students' motivations towards extracurricular research in a Hong Kong university. Drawing upon data from semi-structured interviews, this study analysed the students' motivation change from the cognitive, social, and affective perspective. The findings revealed that the students were externally and internally motivated to join informal research activities and that their motivation change was intertwined with their cognitive learning, social interactions, and emotional experiences in the research process. The paper concluded with practical implications on supporting and sustaining undergraduates' motivations in extracurricular research activities to benefit their academic studies and future work.