The work aims to study effects of micro-diamond particle size on friction and wear properties of Ni-P diamond electroless composite coatings. An electroless composite coatings containing different sizes of micron diamond particles was prepared by adding different sizes of diamond particles in Ni-P plating solution based upon reasonable Ni-P electroless plating parameters. The effects of diamond particle size on morphology and microstructure of Ni-P diamond electroless composite coatings before and after heat treatment were observed and analyzed by using SEM and XRD. The effects of diamond particle size on the friction and wear properties as well as hardness of the Ni-P and Ni-P diamond composite coatings were investigated by performing hardness as well as friction and wear experiments. Thickness of the prepared coatings was about 30 μm and diamond grits were homogeneously dispersed on the coatings with diamond content of 21%~25%. The Ni-P diamond composite coatings were non crystalline structure before heat treatment, and crystallized into Ni3P of higher hardness after 400 ℃ for 2 h heat treatment. The diamond particles could increase hardness of the Ni-P diamond (9 μm) composite coatings to up to 1261HV. Friction coefficient of the Ni-P diamond composite coatings ranged from 0.4~0.52 and decreased continuously as diamond particle size increased. The addition of diamond particles changed wear mechanism of the Ni-P coating. The cemented tungsten carbide ball was worn more seriously as the diamond particle size increased. Both hardness and wear resistance of Ni- P-Diamond coatings increase while friction coefficient decreases as diamond particle size increases.