Positron annihilation lifetime measurements have been carried out for glassy poly(ethylene terephthalate) (PET) as a function of temperature in the temperature range from 15 to 300 K. The structural relaxation and transition were studied by positron annihilation lifetime spectroscopy (PALS). According to the variation of orthopositronium(o-Ps) lifetime with temperature, two intercept temperatures were observed and were designated as the secondary transition temperatures T-gamma (gamma transition) and T-beta (beta transition). The coefficient of thermal expansion of the free volume hole was estimated from the temperature dependence of the free volume. We found that the size of free volume holes does not change significantly with temperature when T < T-gamma, and that it increases evidently with temperature above T-gamma. On the other hand, the distribution of the free volume was examined by developed maximum entropy lifetime method (MELT). Continuous analysis clearly indicated that the free volume distributions were broadened and that the value of peak was shifted toward high value with increasing temperature.