We investigate vanadium (V)-based Ohmic contacts on n-GaN (=$2.010^{18}$ ) as a function of annealing temperature. It is shown that the V (60 nm) contacts become Ohmic with specific contact resistances of Ω upon annealing at 650 and . The V(20 nm)/Ti(60 nm)/Au(20 nm)contacts produce very low specific contact resistances of $2.2 10^{-5}$ and$ 4.010^{-6}$ Ω when annealed at 650 and , respectively. A comparison shows that the use of the overlayers (Ti/Au) is very effective in improving Ohmic property. Based on the current-voltage measurement, Auger electron spectroscopy, glancing angle X-ray diffraction, and X-ray photoemission spectroscopy results, the possible mechanisms for the annealing temperature dependence of the Ohmic behavior of the V-based contacts are described and discussed.d.