A disturbance rejection control scheme based on variable rate reaching law is proposed for Buck converters with matched and mismatched disturbances including input voltage variations, output load mutations, inductance and capacitance perturbations. The variable rate reaching law is constructed by designing an arc cotangent function, such that the convergence rate of the system states is changed to guarantee the fast transient response speed and reduce the controller chattering. By low-pass filtering the system model, a novel disturbance observer is designed to estimate the matched and mismatched disturbances. The developed disturbance observer includes only one design parameter, and the differentiation of the system states is not needed in the disturbance observer, such that the noise amplification problem resulted from the differentiation can be avoided. Finally, the convergence analysis of the observation error and output voltage error is provided based on Lyapunov stability synthesis. Comparative simulations and experiments validate the effectiveness of the proposed scheme.