The damaged depths in GaAs processed by Ar plasma etching under various applied radio-frequency (RF) voltages and Ar pressures in a parallel-plate type apparatus were studied. The damaged depth was evaluated in terms of the chemically etched thickness at which the photoluminescence intensity recovers. The damaged depth decreased as the applied radio frequency peak voltage was decreased. The depth was extraordinarily deep in the lower radio frequency voltage region. The effect of Ar pressure on damaged depth was small in the range from 1 to 10 Pa.