We have previously described an original model called the EVM (energy, volume, and mass) model, which uses only three descriptors, the energy of a polymer segment conformation using molecular mechanics and molecular dynamics, its volume (the occupied space by the atoms as well the unoccupied space between them), and the repeat unit molar mass, to calculate successfully the glass transition temperatures (T-g) of aliphatic acrylate and methacrylate polymers. We report here the application of this model to a series of various polystyrenes. The EVM mode]. allows correct description of the substituent position effect on the ring as well as on the backbone, for various alkyl group and halides. The estimated T-g values with the EVM model are in excellent agreement with Literature values. We compare this model with two other methods (Bicerano＇s model and Porter＇s approach) and find better or comparable correlations.