The index variation of channel waveguides based on a mode field distribution is studied. The measured mode field distributions are compared with the theoretical ones by acquiring the experimental field distributions of the fundamental mode. The core refractive index and its variations of ultraviolet (UV)-written channel waveguides are calculated. Waveguide channels are formed after the UV irradiation of planar waveguides and 1%～3 % index increase occurs in the waveguide core region containing about 20% germanium in mole fraction.