The work aims to study the effects of different diamond particle sizes and contents on the microstructure and properties of diamond/Cu composite coatings prepared by supersonic laser deposition. The microstructure and wear properties of the coatings were analyzed by scanning electron microscopy and friction and wear test. The thermal conductivity of the composite coating was measured by laser scintillation. The diamond particles were evenly distributed in the composite coating. When the content of 800-mesh diamond particles in the original composite powder increased from 30% to 50%, the area ratio of diamond particles in the composite coating only increased from 14.01% to 16.79%, which was much lower than that of the diamond particles in the original composite powder. The average thermal conductivity and friction coefficient of the 400-mesh diamond/Cu composite coating were 296 W/(m?K) and 0.551, respectively; and the average thermal conductivity and friction coefficient of the 800-mesh diamond/Cu composite coating were 238 W/(m.K) and 0.545. The increase of diamond ratio in the original composite powder does not have a significant effect on the increase of diamond content in the composite coating. The thermal conductivity of the diamond/Cu composite coating decreases as the content of the diamond particles increases, and increases as the particle size of the diamond particles increases. The addition of diamond of different particle sizes can significantly reduce the friction coefficient of the Cu coating, and the diamond in smaller size can make the friction coefficient much lower and the coating more stable and then lead to lower wear amount and smaller scar width, so that the excellent wear resistance is obtained.