We measured the positron annihilation lifetime in poly(ethylene terephthalate) (PET)as a function of temperature from 15 to 300 K. From the variation of ortho-positronium (o-Ps) lifetime with temperature, two secondary transition points were observed, i.e.; gamma and beta transitions. On the other hand, the maximum entropy lifetime method (MELT) is used to determine the distribution of the free volume holes. We found that the free volume distribution becomes broad with a raise in temperature. The experimental results indicate that the free volume is changed even if polymer PET is in glassy state.