Positron annihilation lifetime spectroscopy and Fourier transform Infrared spectrometer were used to study the effect of γ irradiation dose on free volume and microstructure of polystyrene-block-butadiene diblock copolymer, respectively. Based on the free volume classic model of Eldrup and examonation of the correlation between their results, the model of γ irradiation effect on cross-linking and degradation of SB is established. Thermal gravity analysis was used to study thermal stability of SB at various irradiation doses to reveal the relationship between thermal stability and micro-structure changes.