Fabrication and eharacterization of proton-exchanged waveguides of MgO: LiNbO3doped with different contents of MgO are reported. The step-like refractive indexprofiles are measured and calculated for Z- cut MgO: LiNbO3 waveguides at differrent wavelengths and the experimental results are shown consistent with each other. Thedispersive relation of refractive index increments is given, and it is shown that the indexincrements and the diffusion coefficients at the same temperature decrease with the content of MgO in the crystal increased. The mechanisms of the effects of MgO on waveguide properties are discussed finally.