Abstract： Positron lifetime and Doppler broadening of annihilation radiations have been measured for PMMA polymer in temperature range from 20℃ to 150℃. The O-Ps annihilation and Doppler broadening data consistently show the structural changes in PMMA. The transitions at 48℃ and 108℃ are attributed to the β transition and glass transition of PMMA respectively. An anomalous change of o-Ps lifetime and intensity around 84℃ is found to be the result of the mergence among some freevolume voids. The results of Doppler broadening measurement also give the information about the variations of degree of structural order. The temperature dependences of size and concentration of free volume in PMMA are estimated from the experimental o-Ps annihilation parameters.