Band alignment between NiO
x
and nonpolar GaN plane and between NiO
x
and semipolar GaN plane are measured by x-ray photoelectron spectroscopy. They demonstrate that the maximum value of the valence band in the unintentional-doped a-plane, m-plane, and r-plane GaN are comparable to each other, which means that all the substrates are of n-type with similar background carrier concentrations. However, the band offset at the NiO
x
/GaN interface presents obvious crystalline plane dependency although they are coated with the same NiO
x
films. By fitting the Ga 3d spectrum obtained from the NiO
x
/GaN interface, we find that relatively high Ga–O content at the interface corresponds to a small band offset. On the one hand, the high Ga–O content on the GaN surface will change the growth mode of NiO
x
. On the other hand, the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.