Copper nanoparticle was prepared by liquid phase reduction technique. Cu2+ was reduced to copper particle by adopting different types of reductants. Ascorbic acid (C6H8O6), phosphinic acid (H3PO2), titanium sulfate (Ti2(SO4)3) and sodium borohydride (NaBH4) were chosen as reductant, respectively. The effect of reaction driving force upon the average size of copper particle was investigated in the paper. It can be concluded that there is a firm relationship between the reaction driving force and the average size of copper particle. The average size of copper particle decreases with the increasing of reaction driving force.