Chinese complementary schools (CCS) play a significant role in transmitting the Chinese language and culture to younger British Chinese generations. Existing research are largely set in Cantonese-dominant CCS, which have characterised long-established UK Chinese communities. However, the last five to ten years have witnessed a demographic change of UK Chinese population and Mandarin is replacing Cantonese as the lingua franca of UK Chinese communities. This research aims to explore the current practices in three CCS and to understand the impacts of UK Chinese demographic change on CCS. Data was collected using ethnographic observation, document collection and semi-structured interviews with teachers, parents and pupils. This research finds that CCS in UK are undergoing great changes, as Mandarin has been the preferred language in these schools, which has implications for the population and activities of these schools. Students’ backgrounds have been extremely diverse, but current resources, materials and expertise of CCS do not address the diversity. Issues and problems arise when regarding Mandarin as the Chinese heritage language of all students attending the schools. The role of CCS in participants’ identity construction is significant as CCS provide an intercultural space for them to reflect on their UK Chinese identities. This doctoral research hopes to provide in-depth insights into Mandarin teaching and learning in CCS and implications for policy makers and practitioners.