By using a new theoretical glass transition temperature (T g)–composition equation, T g’s of statistic binary copolymers obtained from MMA, St and AN were investigated in this article. The copolymers were prepared by bulk copolymerization using azo-bis-isobutyronitrile (AIBN) as initiator. The compositions and T g’s were determined by NMR and DSC, respectively. The monomer reactivity ratios were obtained by nonlinear fitting with Mayo–Lewis equation. Excellent fitting results were obtained when relations of T g’s of MMA–St, MMA–AN, and St–AN copolymers with their compositions were investigated by using a new equation which assumed additivity of bond stiff energy (Liu et al. J Phys Chem B 112:93–99, 2008). This equation contains mole fractions of triads and T g’s of corresponding periodic copolymers. Compared with the widely used Johnston equation and Barton equation, the new equation showed its superiority. Meanwhile, T g’s of the assumed periodic copolymers that have not been acquired were tentatively predicted which may provide useful information.