The invention discloses a smoothing method to decrease bowing of group III nitride semiconductor substrate. The certain face of group III nitride semiconductor substrates is etched under the appropriate etching recipe and time, the certain morphology such as rod-type and other structures are appeared at the certain face. And such structures releases the compressive stresses at these certain faces, resulting in clearly increasing the bowing radius of the group III nitride semiconductor substrates, finally decreasing the bowing phenomenon of the group III nitride semiconductor substrate.