This paper proposes an active-clamp resonant circuit in flyback DC-DC Converter for single phase grid-tied photovoltaic (PV) system. The resonant circuit recycles the energy stored in the leakage inductance of the transformer, limits the voltage stress of the main-switch of the DC-DC Converter during the turn-off process, and provides zero-voltage switching (ZVS) turn-on of the primary switches. Therefore, the switching losses of the high frequency primary switches are negligible. Moreover, the galvanic isolation provided by the high frequency flyback converter improves the reliability of the system. The proposed scheme has been realized by the simulation work and experimentally verified based on a prototype of 250W microinverter.