Informed by Activity Theory, taking a longitudinal qualitative case study approach, this study explored a primary school EFL teacher’s process of conducting action research in an in-service teacher training program. Analysis of multiple sources of data, including interviews, observations, and reflective journals, showed that the teacher’s conducting action research was an expansive learning process full of contradictions. In particular, the interaction with mentors and the outcome of action research largely determine whether the teacher can achieve theory-practice integration and mutual benefit. This study indicated that patient guidance and academic support from expert teachers and university professors, and full activation of teachers’ agency play an important role in primary school teachers’ professional development.