Research on teachers' beliefs has provided useful insights into understanding processes of teaching. However, no research has explored teachers' beliefs as a system nor have researchers investigated the substance of interactions between teachers' beliefs, practices and context. Therefore, the author adopts complexity theory to explore the features of the teachers' belief system and how interactions between different components of the teachers' belief system contribute to complex features of their beliefs. The author illustrates this application by using a case study example of a language teacher in a Chinese secondary school. The study used the methods of semi-structured interviews, observations and stimulated recall interviews. It revealed the co-existence of different types of beliefs. The interaction of these beliefs determined the relationship between the teacher's beliefs and practice. Moreover, the practices of "token adoption" and eclectic approach were noted as the non-linear features of the teacher's belief system at the time of curriculum reform.