A Mg‐ion indiffusion process was applied to the lithium niobate single‐crystal substrates deposited with magnesium oxide thin film. By means of x‐ray diffraction analysis and differential thermal analysis, we find that the magnesium diffused layer exhibits the crystal structure of an unknown compound of the Mg‐Li‐Nb‐O ternary system. It is proposed, that this unknown compound and MgNb2O6are the real sources for the Mg‐ion indiffusion. The changes in Curie temperature with diffusion parameters such as diffusion temperature, diffusion times, and the thickness of magnesium oxide film are noted. ©1995 American Institute of Physics.