The thermodynamics of mechanisms of the reactions in the synthesis processes of WC-Co composite powder by WO3, CO3O4 and carbon as original reactants was investigated. By the thermodynamic calculations, the initiation of the in situ reduction and carbonization reacfions, the formation sequence and the relative stability of the reaction products, can be quantitatively described. The formation sequence of the reaction products, and the fact that WC-Co composite powder with pure phases, homogeneous and ultrafine particles can be synthesized at 1323 K in the vacuum condition, as found in experiments, verified the thermodynamic predictions. The thermodynamic analysis performed in the present work is significant to control the preparation processes and to optimize the parameters of fast synthesizing pure-phased WC-Co composite powder. (C) 2007 Elsevier B.V. All rights reserved.