The relationships between irradiation and B7.1 costimulatory molecule expression of human tumor cells were studied to explore reason of tumor cell's immunogenicity enhanced. The effect of ,γ, rays irradiation on B7.1 molecule expression in human tumor cells which was irradiated with 30,40, 50,60Gy ,γ, rays was investigated. At 24, 48, 72, 96h after irradiation the change of B7.1 molecule was measured by flow cytometry （FCM） with immunofluorescence technique. B7.1m RNA was determined by reverse transcription polymerase chain reaction （RT-PCR）. The cytotoxicity of lymphocyte upon tumor target cell after exposure was measured by ,3,H-thymidine（,3,H-TdR） release assay. The results showed that after γ rays irradiation SMMC-7721 hepatoma,PC-12 pheochromacytoma, A-459 lung cancer cell lines express B7.1 costimulatory molecule. RT-PCR showed that expression of B7.1 molecule in SMMC7721 is resulted from B7.1mRNA expression. Furthermore,B7.1 molecule membrane expression after γ rays irradiation is shown to enhance the cytotoxicity of lymphocyte. The data could explain the enhanced immunogenicity of tumor cells after irradiation, and could lead to new immunotherapy protocols. The experiments suggested that γ rays irradiation could selectively induce human tumor cells to express B7.1 costimulatory molecules and enhance immunogenicity of tumor cell.