In this brief, the problem of voltage regulation of buck converter with a class of nonlinear loads is addressed by using an adaptive energy-based controller. It is noted that the existing results mainly cope with converters feeding one or several nonlinear loads. Other types of loads do exist in power systems. Therefore, based on the passivity theory and a coordinate transformation, this brief proposes a new energy-based controller to stabilize buck converter with a class of nonlinear loads. It is ensured that the closed-loop system is a port-Hamiltonian system with exponential convergence. Then, the estimated terms generated by a designed disturbance observer are forwarded to energy-based controller to reduce the influence of the time-varying disturbances on the system. The experiment test is conducted by means of the setup of buck converter with a constant power load. The simulation and experimental results verify the effectiveness of the designed controller.