Abstract: Ni-based composite electroforming layer with nano-Al2O3 pairticles was prepared under the condition of supercritical fluid, and microstructure and microhardness were analysed by micro hardness tester and scanning electron microscopy. The results show that aggregation problem of nano-Al2O3 particles was resolved effectively by electroforming under the condition of supercritical fluid, and electroforming made nano-Al2O3 particles distribute in electroforming layer evenly. The microhardness of electroforming layer increased and then decreased with the increase of adding amounts of Al2O3 particles. Microhardness of the electroforming layer up to maximum (1 142.2 HV) and mass fraction of Al2O3 in electroforming layer was 9.88wt% when pressure of the supercritical fluid was 14 MPa and adding amount of Al2O3 particles in plating solution was 60 g·L-1.