A dual-reaction model has been proposed to explain the mechanism of direct copper plating on a nonconductor with the assistance of PdS. The reaction in which charge passes through the sulfur ligand is responsible for the lateral growth of copper on the surface. The other reaction is the conventional cupric/copper deposition, which is responsible for the vertical growth of copper deposit. The exchange current density and rest potential of these two reactions have been evaluated via the potentiostatic method. When the [Cu2+] is above 0.3 M, the exchange current densities of these two reactions were found comparable. However, for a low [Cu2+], the i(0) of lateral reaction is significantly higher than that of vertical reaction. Furthermore, the slope partial derivative log i(0)/partial derivative log a(Cu2+) is about 0.58 for the vertical reaction, which is similar to that for the common copper deposition reaction, but is only about one-third of that for the lateral reaction, which implies the occurrence of a different reaction. Regarding the effect of copper concentration on the equilibrium potential, partial derivative E/partial derivative log a(Cu2+) is about 28 mV/dec, which is reasonable based on the Nernst equation. However, in the case of lateral reaction induced by sulfet ligand, the slope is only 18 mV/dec, indicating again the existence of a different reaction.