Generating heterophase structures in nanomaterials, e.g., heterophase metal nanocrystals, is an effective way to tune their physicochemical properties because of their high-energy nature and unique electronic environment of the generated interfaces. However, the direct synthesis of heterophase metal nanocrystals remains a great challenge due to their unstable nature. Herein, we report the in situ and direct synthesis of heterophase Ni nanocrystals on graphene. The heterostructure of face-centered cubic (fcc ) and hexagonal close-packed (hcp ) phase was generated via the epitaxial growth of hcp Ni and the partial transformation of fcc Ni and stabilized by the anchoring effect of graphene toward fcc Ni nanocrystal and the preferential adsorption of surfactant polyethylenimine (PEI) toward epitaxial hcp Ni. Comparing with the fcc Ni nanocrystals grown on graphene, the heterophase (fcc /hcp ) Ni nanocrystals in situ grown on graphene showed a greatly improved catalytic activity and reusability in 4-nitrophenol (4-NP) reduction to 4-aminophenol (4-AP). The measured apparent rate constant and the activity parameter were 2.958 min–1 and 102 min–1·mg–1, respectively, higher than that of the best reported non-noble metal catalysts and most noble metal catalysts. The control experiments and density functional theory calculations reveal that the interface of the fcc and hcp phases enhances the adsorption of substrate 4-NP and thus facilitates the reaction kinetics. This work proves the novel idea for the rational design of heterophase metal nanocrystals by employing the synergistic effect of surfactant and support, and also the potential of creating the heterostructure for enhancing their catalytic reactivity.