This paper proposes a high frequency isolated DC/DC converter, the switching frequency is 20MHz. The system volume can be greatly reduced in such high frequency. Meanwhile, the switch voltage stress is reduced by introducing the third harmonic. In the proposed converter, the switch can operate in ZVS condition and the diode can operate in ZCS condition, which can help to reduce the switching loss. A mathematic model of the proposed high frequency converter is built, which provides an effective design method for the system parameters. To verify the feasibility of the proposed converter and mathematic model, a 10W prototype is built.