Abstarct: To stabilize the output voltage of the photovoltaic systems DC-DC converters usually are used, among these converters SEPIC converter with respect to its features is a suitable option for this work. As this converter be controlled better overall efficiency of system increases, according to the non-linear behavior, existing uncertainties, parametric changes, load disturbances and variable input voltage in DC-DC converters, classic line controllers generally lose their desired and do not show good control functions. H∞ Linear optimal robust controllers show desired functions against unspecified (parametric and non-parametric) and disturbances. This thesis has been presented with the aim of controlling SEPIC converter voltage in the presence of system parametric changes using H∞ robust control method. For this purpose, after the introduction of SEPIC converter, system state-space model and system modeling with an unspecified has been done and H∞ robust controller has been designed to control the output voltage. Then robust controller function results and also compared with PI controller method have been presented. The simulation results show the superiority of H∞ robust controller toward the PI controller.