Traditionally, Hong Kong teachers are trained and performed as subject teachers in the secondary schools. The present study aimed to explore teachers' beliefs about curriculum integration in a secondary school in Hong Kong. Curriculum integration in this particular school meant the fusion of three subjects, i.e. Geography, History and Economic and Public Affairs. Case study was adopted in this study. The unit of analysis was the individual teacher. Multiple methods were used, i.e. documentary evidence, interview and observation. Triangulation of data was ensured. It was found that the teachers generally perceived curriculum integration as a means to solve the problems of the crammed curriculum. Individual teachers' beliefs about integration varied according to their different profiles of professional development. Teachers' implementation was found in accordance with their beliefs. Discussions of the case were in three dimensions, i.e. the influence of school context on teachers' beliefs, the influence of teachers' personal judgments derived from past experiences and the dialectical relationship between teachers' working context and their personal judgments. The results of the study shed light on the literature of teacher development in the upheaval of curriculum innovation. For the policy makers, it is important for them to be aware of teachers' beliefs, which, as the results of the teachers' professional development, were significant factors to the access of curriculum innovation. For the teachers, it is important to help them to be aware of their own beliefs and their respective impact on educational practice.