Effects of thin ZnO/Mg interlayers on electrical and optical properties between p-GaN and ITO were characterized for its application to GaN-LEDs. The ZnO and Mg layers were deposited to have various thicknesses (1~6nm for ZnO and 1~2nm for Mg) by sputtering. After RTA process, the atomic migration between Mg and ZnO and the formation of Ga vacancy were observed from SIMS depth profile, resulting in the increase of hole concentration and the reduction of band bending at the surface region of p-GaN. The sample using ZnO(2nm)/Mg(2nm) interlayer produced the lowest contact resistance with SBH(Schottky barrier height) of 0.576 eV and the transmittance higher than 83% at a wavelength of 460nm when annealed at 500℃ for 3min in air ambient.