Copolymer of poly (L-lactic acid) (PLLA) grafted with N-vinylpyrrolidone (PLLA-g-PVP) was prepared by γ-irradiation in the presence of methanol as a solvent. The effects of acetic acid (HAc) concentration, monomer concentration and dose rate on molecular weight of PLLA-g-PVP were investigated and the structure of PLLA-g-PVP was characterized by ,1,H-NMR, FT-IR and TGA, respectively. It shows that, the HAc concentration, monomer concentration and dose rate have an effect on the molecular weight of PLLA-g-PVP at the same absorbed dose. The maximum molecular weight of the copolymers can be obtained in optimum condition of HAc concentration 0.05mol/L, monomer concentration 30% and dose rate 500 Gy/h, respectively. The two weight loss stages of graft copolymers have been observed more distinctly with the increment of graft ratio.