Glass transition temperature (T_g) is proportional to the quadrupole splitting(Δ) of Fe(III) obtained from the ⁵⁷Fe Mossbauer spectra (T_g-Δ rule (1990)). The values of Δ reflect the distortion of Fe(III) atoms, which occupy the sites of network-forming atoms. Heat treatment of potassium vanadate and calcium gallate glasses at around the individual T_g causes a structural relaxation, accompanying a linear decrease of T_g and Δ values. These experimental results prove that T_g decreases with a decrease in the distortion of VO₄, GaO₄, and FeO₄tetrahedra, as the T_g-Δ rule predicted.