We investigate vanadium (V)-based Ohmic contacts on n-GaN ($N_{d}$=$2.0${ imes}$10^{18}$</TEX> $cm^{-3}$ ) as a function of annealing temperature. It is shown that the V (60 nm) contacts become Ohmic with specific contact resistances of $10^{-3}$ $- 10^{-4}$ Ω$ extrm{cm}^2$ upon annealing at 650 and $850^{circ}C$. The V(20 nm)/Ti(60 nm)/Au(20 nm)contacts produce very low specific contact resistances of $2.2 ${ imes}$ 10^{-5}$</TEX> and$ 4.0${ imes}$10^{-6}$</TEX> Ω$ extrm{cm}^2$ when annealed at 650 and $850{circ}C$, 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.