Power transfer efficiency (PTE) and transferred power (TP) are the key performance indices for the design of wireless power transfer (WPT) systems. An optimization method for WPT system with the use of a repeater is presented to enhance both PTE and TP. A tuning technique is investigated for the performance improvement against load variations. The self-resonance frequencies of transmitter, receiver, and repeater coils, and the operating frequency can be optimized to improve the load characteristics. The analytical approach presented in this paper is found to be capable of reaching the global optimum solution for a given design condition. Theoretical analysis is supported with the numerical and experimental results.