A GaN reduced surface field (RESURF) metal oxide semiconductor (MOSFET) on a sapphire substrate is fabricated. The n --type RESURF zone was formed by a Si ion implantation technique. The n +- and n --type GaN was activated at 1260degC for 30degs in ambient Ar. The sheet carrier densities (activation ratio) of n +- and n --GaN were ~3.0~1015 (~100~) and 1.1~1012 cm-2 (1.8%), respectively. As a result, more than 1500%V and 2%A operation of the GaN RESURF MOSFETs is achieved with a channel length of 4%%m, a channel width of 150%mm, and the RESURF length of 20%%m.