Ceramic alloys were synthesized by reaction sintering and densified by HIP in the temperature range of 2070 to 2120K under a pressure of 200MPa. The starting powders were commerical silicon nitride, aluminium nitride and graphite powders. A small amount of calcia was added in the form of Ca(NO3)2⋅4H2O, and the reaction sintering was performed in a nitrogen atmosphere in the temperature range of 1670 to 2170K. The reaction process was investigated by X-ray diffraction analysis and found to be consisted of two steps: the formation of α-sialon and its carbothermal reduction. When AIN content was less than 35mol%, a uniform 2HSiC-AIN solid solution was formed with fine grain size of about one-third of that of the strating powders. On the other hand, when more AIN powders was added, two-phase composite was formed. Complete densification of the synthesized compact was attained by HIP treatment.