A method of designing a repetitive-control system based on a reduced-order extended-state observer (ROESO) is presented for a single-phase full-bridge LC-type inverter subject to parameter perturbation and load disturbances. First, the mathematical model of the inverter is derived from the circuit theorem. An ROESO is constructed by making use of the measurable output voltage of the system to estimate the total disturbance, including the parameter uncertainty and external load disturbance in a real-time fashion. Next, a plug-in modified repetitive controller is inserted into the closed-loop system and based on the disturbance estimate, a composite control law is constituted, which effectively attenuates the influence of the total disturbance on the output of the system and ensures the high-accuracy tracking of the periodic reference input. Then, the global stability condition of the closed-loop system is established using the small-gain theorem. The parameter regulation method and design procedure of the system are given. Finally, simulation results demonstrate the validity and superiorities of the method.