A method of index measurement, the so-called rn-line technique has been applied to measure the refractive index of planar waveguides fabricated by ion exchange method in BK7 substrate. By placing a prism coupler on the surface of the planar waveguide, the coupling angle of modes guided in the waveguide was measured. The prism has apex angle of 44.9° and is made of ZnSe with refractive index 2.59073 . The values of the coupling angles were then processed mathematically to obtain the effective indices of the guided modes. The number of modes guided in the waveguide depends on the duration of ion exchange process, and the effective refractive indices have been determined for the respective modes. The result for zero order mode, ranging from 1.5183 to 1.6887 for TM modes and from 1.5182 to 1.6891 for TE modes. On the other hand,for the duration of ion exchange process of 48 hours, five modes were guided in the waveguide and the effective refractive indices were 1.6887; 1.6167; 1.5818; 1.5649 and 1.5465 for zeroth, the first, the second, the third and the fourth modes, respectively. The use of the rn-line technique has been proved to be simple and effective with high accuracy in the characterization purpose of waveguides.