Herein, ZrC powders with the average particle sizes of 100–300 nm were synthesized via a method composed of vacuum carbothermal reduction followed by calcium treatment. The effects of carbon and calcium addition amounts as well as temperature on the micromorphology of obtained products had been studied, and the involved mechanisms were described. Due to the dissolution-transportation-precipitation mechanism, ZrC grains would grow during the calcium treatment process. The finest ZrC with the average particle size of 126.2 nm could be obtained by the combination of vacuum carbothermal reduction at 1773 K for 4 h and calcium treatment at 1173 K for 4 h, with the carbon addition amount of 1.2 times the theoretical value. Furthermore, carbon defect was found in the final products and the lattice spacing was in good agreement with those calculated by proposed formula.