In this paper, an optimal controller is proposed to control the voltage of photovoltaic system as distributed generation (DG) using intelligent algorithm in islanding mode operation. The control algorithm used in the studied control plant during load variations is based on the reinforcement learning (RL) algorithm. To control the examined plant, this algorithm has tried to prevent controller output voltage changes due to the load variations by changing the constant and integral coefficients of load variations, and to keep the output voltage of photovoltaic system in certain constant value in the islanding mode. The proposed controller performance to achieve this goal is verified by the simulation results.