Extensive application of GaN-based blue LED is greatly affected by increase in light-extraction efficiency. Refraction index of Ohmic contact to p-type GaN developed by oxidizing Ni/Au thin films is calculated. Power flow propagation within the p-contact is analyzed and coupling layer with proper refraction index over the p-contact is design to reduce light-absorption and reflection by p-contact. Frustum of cone structure scheme of ITO layer is applied to attenuating total reflectance(ATR)of the interface between air and ITO so that light-extraction efficiency of GaN-based LED can be increased. Result calculated by transfer matrix method shows that transmissivity of coupling layer/p-contact films at 450 nm wavelength increases to 75% for the ITO coupling layer with optical thickness of π/2 and refraction index of 2.02.