Abstract: The diffusion coefficients of Fe²⁺ and Ti⁴⁺ and their influence on the process of diffusion treatment of sapphire are discussed. According to the thickness of the diffusion layer and the process of the diffusion-treated sapphire from Thailand, the diffusion coefficients of the Fe²⁺ and Ti⁴⁺ are computed. Experiment shows that diffusion coefficient of the colouration ion is important in working out the diffusion process.