Spherical tungsten powders with the average grain size of 56.4 nm were prepared by hydrogen reduction assisted by carbon because the volatility of tungsten oxide, which is the fundamental reason of the rapid and abnormal grain growth of tungsten powder during hydrogen reduction, was inhibited by carbon addition. The effect of carbon on the reduction mechanism of tungsten oxide was investigated. It is shown that the C:W molar ratio of precursors has a great influence on the grain size and residual carbon content of tungsten powders, and the optimum molar ratio is 2.6. The grain size increases with the prolongation of reduction time, and the grain growth rate is closely related to the reduction temperature. With the reduction temperature increase from 680 ℃ to 760 ℃, the grain size and residual carbon of tungsten powders decreases and the dispersion is improved, and then the grain size increases slightly when the reduction temperature increase to 800 ℃. The results indicate that the reaction between carbon and H2O steam occurs when the reduction temperature exceeded 710 ℃, which can reduce the partial pressure of H2O steam. Therefore, the volatility of tungsten oxides is inhibited, and the dominant growth mechanism of tungsten powders transforms from volatility-sedimentation mechanism to atomic diffusion mechanism.