Display omitted •Graphene reacted with Ti matrix to form TiC nanotwin during friction stir process.•Stir zone near surface contained more TiC and obtained higher hardness.•Fewer slip systems in stir zone were confirmed by in situ microcompression tests. Gradient materials have significant potential to break strong plastic tradeoffs. Graphene with a strong affinity for titanium alloys has an influential application value for material modification. In this study, microstructure evolution and deformation behavior of graphene-based TiC/Ti6Al4V composites processed by friction stir processing (FSP) have been investigated. Electron backscattered diffraction (EBSD) reveals uniform microstructure in the stir zone (SZ). Transmission electron microscopy (TEM) observations reveal distinct microstructures at different depths from the processed surface. The SZ includes nano/micro grains and TiC nanoplates. Twin structure exists in both α matrix and TiC. Stress-induced martensitic transformation is suppressed. As depth increases, TiC gradually disappears and the FSP-induced texture {-2116} < 2–1–11 > becomes slightly stronger. Moreover, there exists special crystallographic orientation relation: (111)TiC//(0001)Ti. In the base metal (BM), larger grains are observed, and dislocation structure becomes the dominant defect feature. Nanoindentation results show that hardness decreases first and then increases from the processed surface to the bulk metal. The distribution of hardness is the result of combined action of strengthening effect of TiC twins and deformation adaptation effect of α twins.