We report the value of surface band bending for undoped, a-plane GaN layers grown on r-plane sapphire by metalorganic vapor phase epitaxy. The surface potential was measured directly by ambient scanning Kelvin probe microscopy. The upward surface band bending of GaN films grown in the  direction was found to be 1.1±0.1 V. Because polarization effects are not present on a-plane GaN, we attribute such band bending to the presence of charged surface states. We have modeled the surface band bending assuming a localized level of surface states in the band gap on the surface. It should be noted that the band bending observed for a-plane layers is comparable to that obtained on polar c-plane layers, and both a-plane and c-plane GaN films with similar surface treatments demonstrate comparable band bending behavior, indicating that charged surface states dominate band banding in both cases.