The effects of different WC particle sizes on WC/10Co injection molding process (PIM) were investigated in this study. The influence mechanisms of WC particle size on feedstock flowability, molded compact’s quality, degreasing process and sintering process were analyzed. The results show that as the WC particle size decreases, the flowability of WC/10Co feedstock, the molded compacts’ quality and the dimensional accuracy decrease, only the solvent degreasing rate increases. For the ultrafine WC/10Co feedstock with powder agglomerated particles and insufficient binder coating, the heat stability is lower and the flowability reduces by 56%, which lead to the density of molded compacts reduced by 5%. As to as-sintered ultrafine WC-10Co which is prone to be decarburization during degreasing-vacuum sintering, the line shrinkage is 20.80% and the dimensional deviation is 2.85%.