In view of bond rotation flexibility and the additivity of stiff energy, a new equation containing mole fractions of sequences and glass transition temperatures (T_g's) of periodic copolymers was proposed. Meanwhile, the effect of stereochemical sequences was also taken into account. Even for the case of all substitutions on primary (C₁), second (C₂), and third (C₃) carbon atoms adjoining to the given bond (triad sequences effect) to be considered, no intractable parameters exist in the new equation if the periodic copolymers of poly[AB], poly[AAB], and poly[BBA] could be acquired. This may facilitate its convenient application in comparison with the Ham equation (Ham, G. E. J. Macromol. Sci. Chem. 1975, A9, 461), which also concerned triad contributions. The new equation was applied to methyl methacrylate-styrene, ethylene-methyl methacrylate, and ethylene-vinyl acetate copolymers, and excellent fittings were obtained. The parameters obtained may predict T_g values of periodic copolymers that have not been acquired as well as provide useful information on the bond rotation flexibility.