Ceramic composites containing BN and Al can be machined easily into complex shape, the hardness of the composites can also be improved by surface hardening process of in situ reaction between Al and BN. However, the reaction of Al and BN is a volume reduced one and the porosity will increase during the hardening treatment.. In order to solve this problem, a surface nitriding process was developed. Put the pre-sintering samples (with designed shapes) into a vacuum furnace in nitrogen atmosphere and reheated to high-temperature, so that the residual Al after pre-sintering would react with N2 completely, and change to AlN which has higher hardness. The phase transformation and microstructure were observed and the mechanical and other properties were also measured. The results showed that the surface nitriding process could increase the hardness, bending strength and density obviously. The porosity decreased sharply compared with the general hardening treatment.