The theoretical and experimental studies of anharmonic temperature factor in X-ray and neutron diffraction are reviewed since early works in 1963. These studies are in the framework of effective one body particle potential in which atoms are treated as independent oscillators.The experimental works are classified into two major groups : investigations of anti-centrosymmetric anharmonicity and centrosymmetric anharmonicity. It is pointed out that anti-centrosymmetric anharmonicity is well determined from the accurate Bragg intensity data at a certain temperature but centrosymmetric anharmonicity, because of the correlation of harmonic and anharmonic potential parameters, both of which are centrosymmetric. It is more reliable to determine the centrosymmetric anharmoni-city using the temperature dependence of integrated Bragg intensities unless extremely high Q data are collected.It is suggested that the degree of anharmonicity is much betterdescribed by a ratio of anharmonic term to harmonic term in one body particle potential than the magnitude of anharmonic potential parameter itself. The anharmonic effect is more significant for those substances which have smaller anharmonic potential coefficients.The relationships between anharmonicity of temperature factor and structural phase transition is discussed in the case of several perovskite substances.