The o-Ps lifetime tau(3) and the intensity I-3 of ST-AN copolymers and ST-MMA copolymers have been determined by using the positron annihilation technique. The average fi ee volume hole radius R is estimated according to Tao＇s and Eldrup＇s model. The result shows that the average free volume hole size mainly attributes to lateral group volume and polarity of macromolecular chain as well as polymerizing temperature, and the o-Ps intensity I-3 to the effect of the lateral group volume and the polarity.