In this study, we reported a decrease of oxygen concentration and an increase in the growth rate of a-plane gallium nitride (a-GaN) film grown using Ga2O gas and NH3 gas. The oxygen concentration in a-GaN film was decreased with increasing partial pressure of NH3 and growth temperature. An a-GaN film with the lowest oxygen concentration of 4 ×1018 atoms/cm3 and the growth rate of 18 µm/h was obtained under NH3 partial pressure of 84 kPa at 1250 °C. We concluded that growth under high partial pressure of NH3 at high temperature can produce a-plane GaN film with a high growth rate and low oxygen concentration.