Positron annihilation lifetime spectroscopy was used to study e+ self -irradiation effect of 22Na on positron annihilation parameters of SEBS, tem perature dependence of free volume hole radius and free volume hole fractional, based on the classic model of Eldrup, presenting the linear variation of fractional free volume versus temperature in vacuum the range below Tg and ab ove Tg, respectively. We also discuss the thermal expansion coefficient a nd the constant A in free volume fractional formula by combining the free volume theory of WLF and the classical model of Eldrup.