This paper proposes an integral SMC for the regulation of output voltage of DC to DC buck converter under unknown load changes. The conventional SMC can handle the matched disturbance alone. In buck DC to DC converter the disturbances due to load change appear as mismatched uncertainty. In this paper, the state space averaged model of buck converter is used to design the controller and the model is feedback linearized using a state and input transformation to simplify the design of controller. The sliding surface is designed with integral of output voltage error in addition to output voltage error in order to alleviate the effect of load change in the output voltage of converter. The performance of buck converter using the proposed controller is validated by numeric simulations. The comparison of the results with the basic SMC shows that the proposed integral SMC alleviates the effect of mismatched uncertainties in the output voltage. © 2019 IEEE.