The effect of drawing and heat treatment for PET chip and filament on the thermal characteristics, long period and fine structure were studied. Abnormal volume expansion displayed at around 200$^{circ}C$ corresponds with works by Ueberreiter. Determination of specific heat and D.T.A. thermogram provides endothermic changes to show the abnormal volume expansion. In the course of gradual increasing of temperature (0.5$^{circ}C$/min), long period shows minimum value of 100${AA}$ near 140$^{circ}C$ and it increases up to 125${AA}$ representing thickening of the crystal lamella with increasing temperature up to 220$^{circ}C$. Thickening mechanism of the lamella can be explained by the sliding diffusion theory, however, it is considered to be reasonable to interpret it as the molecular fractionation. Crystalization energy decreases with increase in draw ratio and deflection point can be found at draw ratio of 2.2, where crystallization energy decreases rapidly. Two peaks are shown at the D.T.A. thermogram of the melting point. The peak of the lower temperature one can be considered as the unstallized part susceptible to be rearranged.