Abstract: Hot dipped galvanized (HDG) low carbon steels have been widely used in internal and outside plates of auto body. Constant electrode force was utilized under conventional pneumatic guns, which led to unstablewelding quality.However, the quality of resistance spot welding (RSW)on galvanized steels was sensitive to variable electrode force.Considering controlling electrode force exactly with servo guns, a design of experiment (DOE)method was applied to analyze the influence of three stages of electrode force during the welding process, including preliminary force, welding force and forging force, on welding quality and to obtain optimum parameters of variable electrode force.The results showed that forging force was noted as the most important factor, welding force as secondary one, squeeze force as least important one, and that optimum parameters could increase tensile-shear force and nugget diameter of welds significantly.This method would be the foundation of real-time quality evaluation and control based on electrode force.