The high hardness Ni-P-α-Al2O3 nano-composite coatings were prepared on brass substrates with the composite electroplating technology. The effect of cathodic current density, nano-α-Al2O3 content, pH value of plating bath, temperature of plating bath and plating time on the hardness of coatng was studied. The results show that the optimization conditions are the temperature 45℃, the cathodic current density4 A/dm2, the pH value 4.0 of the bath, the time 40 min and nano-α-Al2O3 content 10g/L. Under these conditions, the coatings are uniform and smooth. After appropriately heating treatment, the microhardness achieves 1 332HV.