The thermal annealing effects on Ti/Al/Ni/Au Ohmic contact to n-GaN are investigated by current-voltage (I-V) characteristics and transmission line method (TLM) measurements. The cladding layer of Ni/Au on Ti/Al plays two roles: preventing inter-diffusion of Ti, Al, Au and anti-oxidation of the contacting layer. The specific contact resistance (ρ c ) of Ti/Al/Ni/Au to n-GaN increases slightly at first with the increasing annealing temperature (T a ). When T a increases above 500 o C, ρ c decreases monotonously in the range of 400-900 o C. However, the morphology of the contact degrades gradually when T a increases above 600 o C. The minimum of ρ c is obtained as 9.65x10 - 7 Ωcm 2 by two-step annealing method in this work. Finally, the roles of two-step annealing method in the formation mechanism of the Ohmic contact to n-GaN are also discussed.