Change of free volume holes during crystallization of amorphous poly(ethylene terephthalate) at 130°C was investigated by positron annihilation lifetime technique together with the measurements of specific density and wide angle x-ray scattering (WAXS). The o-Ps intensity, I3, decreased by 24.6% with the crystallization. This value agreed with the degree of crystallinity determined by the specific density and WAXS, which indicates that o-Ps annihilation takes place in the amorphous part. The average o-Ps lifetime increased slightly by the crystallization. Detailed analysis of the o-Ps lifetime by a continuous distribution model showed that the small size component of the distribution disappeared due to the crystallization. The result is favorably compared with the recently proposed picture of crystallization which deals with the conformational change of ethylene group.