With the conventional X-ray diffraction and grazing-incidence X-ray diffraction. We have investigated the MgO-rich layer characteristics in the magnesium diffused lithium niobate single crystal, when the magnesium-ion indiffusion process in the layer is carried out in atmosphere. It is found that the MgO-rich layer exhibits the crystal structure of Li3Mg2NbO6 and Mg4Nb2O9, when indiffusion time for magnesium-ion is short. these aspects indicate that Li3Mg2NbO6 is in the subsurface layer of MgO-rich layer and Mg4Nb2O9 forms an uniform layer in the MgO-rich layer.When further increasing diffusion time the lithium-ion outdiffusion from the bulk of lithium niobate become prominent and Li3Mg2NbO6 and Mg4Nb2O9 compounds disappear in succession, and LiNb3O8 compound structure appears consequentially. It is observed by scanning electron microscopy that the surface of magnesium diffused lithium niobate indicates a multicrystal structure, the decomposing and disappearing processes of these surface layer compounds are also revealed