The electrical and optical performances of multiple-quantum-well (MQW) InGaN/GaN light-emitting diodes (LEDs) were improved by using chlorine to treat the surface of the p-type GaN layer. The chlorine was produced from electrolyzing diluted HCl(aq). The chlorine reacted with the p-type GaN surface and induced Ga vacancies in the surface region. The specific contact resistance of 4.8 x 10(-6) Omega cm(2) was obtained for Ni/Au metals contact with the chlorine-treated p-type GaN due to the creation of more hole carriers via the inducement of Ga vacancies. Compared with the untreated LEDs, the current-voltage (I-V) characteristics showed that the forward voltage of the chlorine-treated MQW InGaN/GaN LEDs decreased from 3.3 to 3.0 V at a driving current of 20 mA, and the light output power increases 1.25 times at 300 mA. The reverse leakage current of the chlorine-treated MQW InGaN/GaN LEDs was also significantly decreased due to the passivation of surface states by chlorination treatment of p-type GaN layer. (c) 2007 The Electrochemical Society.