nowadays, many researches for plug-in hybrid electric vehicles have been actively carried out to improve the gas mileage in comparison with mass-produced hybrid electric vehicles. In this paper, the on-board charger for plug-in hybrid electric vehicles is studied for obtaining the high efficiency. The on-board charger consists of two phase interleaved PFC circuit and LLC resonant converter. The new design procedures of LLC resonant converter are proposed in this paper. These are very simple and powerful method. In order to verify the abovementioned contents, the LLC resonant converter is designed and tested by using PSIM tool.