In this paper, GNPs/Al composites were fabricated by friction stir processing(FSP) used pure aluminum as the matrix materials. The microstructure (including the matrix, reinforcement and interface) and mechanical properties were investigated, and the reinforcing mechanism of the composites was analyzed. The results showed that the matrix was?obviously refined, and many high angle grain boundaries were generated. The GNPs tended to be exfoliated and broken resulting in large amounts of GNPs edges during FSP. The Al-C atomic-sacle diffusion is easy owing to C atoms activity at the GNPs edges, as a result the interface transitions were formed between the GNPs edges and Al matrix. The yield strength and ultimate tensile strength of 1.8 vol% GNPs/Al composites were 72MPa and 147MPa. Compared to that of unreinforced Al, they increased by 89.5% and 79.3%, respectively. One by one, interface load transfer, Orowan and grain refinement were the principal reinforcing mechanism according to the theoretical calculation，and the calculated and experimental yield strength of the composites increase with the graphene content, concurrently, the deviations between the experimental and calculated strengths also slightly increased. However, the interface load transfer can not be well achieved owing to GNPs disorder distribution in the composites, therefore there is still a gap between the actual yield strength and theoretical calculation.