The heat capacity of 21 kinds of glassy polymers, mainly synthesized in our laboratory, was measured by differential scanning calorimetry. Glass transition temperature (T-g) and the heat capacity difference between the molten state and the glassy state (Delta C-p) at T-g were evaluated. A linear relationship was obtained between Delta C-p and T-g for almost all glassy polymers including engineering plastics, in a temperature range from 230 to 520 K, except for those having large side chains and/or inter-molecular hydrogen bondings. It was suggested that configurational entropic factors must be taken into consideration for analyzing glass transition.