The paper is focused on understanding the densification characteristics of WC-Co composite powders by the spark plasma sintering (SPS) technique, especially the essential mechanisms for the distinct differences of the effects of WC particle size on the densification behavior between the SPS and the conventional sintering technologies. For the particular combination and contacting state between WC and Co after ball milling in which Co forms thin films coating the WC particles, a model that quantitatively describes the densification process of SPS WC-Co powders with different WC particle sizes has been developed. The calculated results show that both the actual temperature in the Co film and the melting temperature of the Co film increase with the increase of the WC particle size. As a result, the formation and growth of the sintering necks due to the rapid melting and solidification of the Co films turn to be weakly influenced by the WC particle size, and hence the SPS densification is almost independent of the WC particle size. The model calculations are consistent with the experimental findings that in the SPS processes the temperatures corresponding to the start of the densification and the peak of the displacement rate, respectively, are nearly the same for the WC-Co powders with different WC particle sizes.