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Photoassisted electrochemical (PEC) etching of n-doped GaN layer grown on sapphire in the KOH based solution under illumination by a mercury-xenon-arc lamp is demonstrated. Smooth surfaces were obtained for a narrow range of etching conditions. It was found that this range could be extended by using etch conditions which produced "whiskers". Subsequent post treatment in developer AZ 400K and KOH solution were used to remove these whiskers and reduced fibrous texture of the grooves walls. This can provide a smooth sidewall on the PEC etched surface. As a result of optimized PEC process 500 nm wide grooves in GaN were successfully obtained.