The influence of the chain extension in the glassy state on the glass transition temperature was investigated for poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) fibers. The extended samples were prepared by drawing at just above the glass transition temperature and subsequently cooling to the room temperature under a constant length condition. The glass transition temperature was measured using differential scanning calorimeter without externally applied stress to the sample. The depression of glass transition temperature with increasing the extension ratio was observed for both PET and PEN fibers. An equation for the depression of glass transition temperature is derived by modifying the DiMarzio＇s theory and applied to the experimental data. In lower extension for PET fiber, the equation predicts successfully the depression of glass transition temperature in accordance with the empirical result. However, for the PEN fiber and the highly stretched PET fiber, the empirical data deviates from the prediction, because of the crystallinity increment of the fiber during the drawing process.