The extent of subsurface damage on (0001) GaN wafers post different polishing treatments was quantified using depth-resolved cathodoluminescence spectroscopy (DRCLS). The band edge emission spectra were obtained from CLS with different electron energies, which manifested a significant non-radiative recombination resulted from polishing-induced subsurface damage. Cross-sectional transmission electron microscopy (XTEM) was also used to diagnose the extent of the subsurface damage layer. For the GaN polished with 1.00 and 0.25 mu m diamonds abrasive, the extent of non-radiative subsurface damage is about 250 and 100 nm, corresponding to the calculated electron penetration depth at the accelerating voltage for the onset of band edge emission. In this study, the depth of subsurface damage estimated from CL spectra compared well with direct XTEM measurements in GaN substrate.