Abstract: A design optimization scheme is proposed to optimize both the number and locations of the gates by analyzing the filling process,in order to reduce the part warpage and weld line, numerical simulation of injection mold filling process is combined with the design optimization method to find the optimum number of gates and their locations to achieve balanced flow and less weld lines while satisfying the limit of injection pressure. The objective function is expressed as the sum of the differences of the maximum and minimum times of boundary filling and a penalty item. The coordinates of gate locations are chosen as design variables. The constraint is to limit the inlet pressure to remain below reference value and side constraints to limit the gate locations to a specified region of the mold. The number of gates is determined in the re-analysis of the filling process. A genetic method is implemented as the optimization algorithm. Numerical examples are given to demonstrate the effect of optimization method.