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WC-6Co nanocrystalline composite powders without excess C and decarburization phases were synthesized via a facile route including spraying conversion, calcination and in situ reduction-carbonization processes. The phase constituents obtained by XRD showed that the as-prepared powders after each step of the preparation were amorphous, WO3 and Co3O4 phases, WC and Co phases, respectively. The pure WC-Co composite powders could be obtained by a reduction-carbonization process heat treated at 900 °C for 1 h under a hydrogen atmosphere due to the catalytic effect of Co on the carbonation reaction. The effects of heat treated temperature on phase constituents of powders were investigated in a range of 700-900 °C. Morphology and microstructure of powders were observed by SEM and HRTEM. The results indicated that the morphology of powder was spherical. The grain size of WC was about 0.36 μm, and the crystalline size of WC was about 56 nm, which indicated that the WC grain was polycrystalline and comprised of many crystallites. It was also found that the individual particles of WC were bonded together under the action of Co. Furthermore, a few sintering necks could be observed in the composite powders due to the contacts between WC particles. The stoichiometry of WC-Co composite powders could be adjusted easily. Moreover, the formation process and mechanism of the sphere structure are also discussed in this paper.