In this papot we present a new method for measurement of refractive index of LiNbO3 optical waveguides, i. e. we can calculate the refractive index of the substrate by the angle, at which the light intensity reflected from the bottom face of the coupling prism is falling down abruptly due to the destruction of total internal reflection. This method can be used to check the usage feasibility of LiNbO3 substrate.