Ti(24 nm)/Al(90 nm) film was deposited on unintentional doped GaN and Si-doped GaN to form metal/semiconductor contacts ,which were annealed at 400～800℃ in the ambient gas of N2.The result shows that Schottky barrier height decreases , ideal factor increases and surface condition of contacts degenerates when annealing temperature increases for unintentional doped GaN. The sample at 600℃ annealing forms Ohmic contacts with low specific contact resistance of 3.03×10~(-4) Ω cm~2. However, the sample for Si-doped GaN with carrier concentration of 5.88×10~(18)cm~(-3) can form Ohmic contacts without annealing, specific contact resistance of the sample is 4.03×10~(-4)Ω cm~2.