The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 0.67 to 0.056 m -1 (i.e. the bowing radius increased from 1.5 to 17.8 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face, and eventually changed the bowing direction from convex to concave. Furthermore, the influences of the bowing curvature on the measured full width at half maximum (FWHM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced, which reduced from 176.8 to 88.8 arcsec with increase in ICP etching time. Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V Ga O N complex defects on removing the GaN layer from N-polar face, which removed large amount of defects, was one of the reasons that improved the bowing of the free-standing GaN substrate. Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching, which released the compressive strain of the free-standing GaN substrate. By doing so, crack-free and extremely flat free-standing GaN substrates with a bowing radius of 17.8 m could be obtained.