Recently GaN-based high electron mobility transistors(HEMTs)have revealed the superior properties of a high breakdown field and high electron saturation velocity.Reduction of the gate leakage current is one of the key issues to be solved for their further improvement.This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers,and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current-voltage(I-V)measurements.A non-linear fitting method Was used to extract the contact parameters from the I-V characteristic curves.Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing.And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current,respectively,can be obtained under 12 rain annealing at 450℃ in N2 ambience.