Lattice parameters, ferroelectric Curie temperature (Tc) and density of sintered MgO doped LiNbO3 were evaluated for investigating the substitution mechanism of Mg in LiNbO3 crystal. The variations in the hexagonal lattice parameters, Tc and the density of specimens were changed with MgO concentration. It is likely that the substitution of Nb on the Li-sites by Mg occurs dominantly below 4 mol% MgO concentration, then substitution of Li by Mg begins to occur above 4 mol%.