The current collapse phenomena in 380V/1.9A GaN power-HEMTs designed for high-voltage power electronics application is reported. The influence of these phenomena to the power-electronics circuit performance under high applied voltage is discussed using a 27.1 MHz class-E amplifier, which can be one of an industrial application candidate. It has been found that the optimized field plate structure minimizes the increase of conduction loss caused by the current collapse phenomena and thus improves the power efficiency of the circuit. The minimized device achieved the output power of 13.8 W and the power efficiency of 89.6 % for the demonstrated circuit even with the applied drain voltage of 330 V and the switching frequency of 27.1 MHz. These results show the future possibility of a new GaN-device application with both high voltage and high frequency condition.