Equilibrium phase composition at 1 atm total pressure is quantitatively calculated as the function of starting materials composition for carbothermal nitridation synthesis of Si3N4/SiC composite powders. N-2 flow is simulated by the N-2 amount in the starting materials. At low starting N-2 content (molar ratio N-2/SiO2 less than or equal to 100), Si3N4 and SiC can not exist in equilibrium at the same temperature. At higher starting N-2 content, the Si3N4-SiC coexisting temperature range appears above 1760 K. The upper limit of the temperature range becomes higher with increasing starting N-2 content. Starting carbon content affects the composition in the equilibrium powder products. The calculation results are compared with experiments.