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In this report, a cost-effective photoenhanced electrochemical (PEC) process was proposed to assist in forming a deep undercut sidewall and rough surface on a GaN light-emitting diode (LED). As a result, a 51% improvement in light extraction efficiency was obtained compared to a standard LED without the PEC process. Manipulating the direction of light illumination during the PEC process produced an undercut angle and rough surface on the sidewall of GaN LED, which enhanced light extraction by about 14% compared to a sample prepared using the conventional PEC process. A simulation based on a two-dimensional finite-difference time-domain method was established to investigate the light output intensity for LED samples by manipulating different illumination directions in the PEC processes. Simulation results indicated that photons have a higher probability of being emitted from the undercut sidewall and cone-like surface of the LED structure formed by the PEC process; this improved the light extraction efficiency of GaN LED.