The present invention relates to a high reliability field effect power device and a manufacturing method thereof. A method of manufacturing a field effect power device includes sequentially forming a transfer layer, a buffer layer, a barrier layer and a passivation layer on a substrate, patterning the passivation layer by etching a first region of the passivation layer, and forming at least one electrode on the first region of the barrier layer exposed by patterning the passivation layer, wherein the first region is provided to form the at least one electrode, and the passivation layer may include a material having a wider bandgap than the barrier layer to prevent a trapping effect and a leakage current of the field effect power device.