A study of the Mg-ion indiffusion of lithium niobate single crystal substrate is made by MgF2 as a diffusion source. By means of electron probe microanalysis, the Mg-ion concentra tion of the Mg diffused layer is obtained, which is approximately a half parabolic and step profiles. The activation energy of Mg-ion indiffusion is 104 kJ/mol. It is found by X-ray diffraction analysis that in the Mg diffused layer LiNb3O8 compound appears with Mg-ion in diffusion beyond a certain extent. The appearance of LiNb3O8 compound is related to the Li ion outdiffusion from the bulk of lithium niobate and existence of a great deal of F-ion at the diffused surface. It is observed by scanning electron microscopy that the F-ion is in the form of LiF and LiF will go away from the surface of the Mg diffused layer with increasing of Mg ion indiffusion degree.