Resonant dc-dc converters have been applied to many industrial applications. In order to dc-dc converter operates in fixed frequency and the zero-voltage-switching (ZVS) condition per different loads, using robust control theory has been reported. In this paper, an optimal sliding mode controller for dc-dc series resonant converter is proposed to control the output voltage of converter in presence load varying. Modeling of converter contains three stages: state-space model, transformed model, and averaged model. This model is suitable for designing the sliding-mode controller. To optimize the controller genetic algorithm is employed. Simulation results are shown in order to validate the proposed optimal sliding mode controller.