In this paper a new application of PSO technique for optimizing the controller parameters of multi modular converters is presented. PSO based LQR controller is designed for averaged model of modular converter and to obtain better control parameters though the minimizing a quadratic-performance index. In this method in order to have more accuracy and better design for controller parameters, PSO algorithm is added to find the gain matrix of the controller. Simulation results show that with using this method better dynamic responses are accessible.