We analyzed the effects of the interface trap on the output characteristics of an inversion mode n-channel GaN Schottky barrier (SB)-MOSFET based on the Nit distribution using TCAD simulation. As interface trap number density (Nit) increased, the threshold voltageincreased while the drain current density decreased. Under Nit=5.0×1010 cm-2 condition, the threshold voltage was 3.2 V for VDS=1 V,and the drain current density reduced to 2.4 mA/mm relative to the non-trap condition. Regardless of the Nit distribution type, there wasan increase in the subthreshold swing (SS) following an increase in Nit. Under U-shaped Nit distribution, it was confirmed that the SSvaried depending on the gate voltage. The interface fixed charge (Qf) caused an shift in the threshold voltage and increased the off-statecurrent collectively with the surface trap. In summary, GaN SB-MOSFET can be a building block for high power UV optoelectroniccircuit provided the surface state is significantly reduced.