Objective To study the effect of three types of size of silver nanoparticles on the effect of enhancing radiosensitivity of glioma cells. Methods 20, 50 and 100nm silver nanoparticles are all prepared in sodium citrate system and were characterized by SEM and TEM. The cytotoxicity was tested and the proper dose was determined to guarantee the little effect on cell viability. The effect of silver nanoparticles on radiosensitivity of glioma cells was detected by MTT assay and colony-forming assay. The cell cycles and apoptosis after irritation were tested by FCM. Result 20nm silver nanoparticles significantly sensitized the effect of IR treatment upon cancer cells. 50nm silver nanoparticles has less effect and 100nm silver nanoparticles has little. Conclusions Small size of silver nanoparticle could efficiently enhance cytotoxicity of irradiation of glioma， which provide new prospect for clinical therapy.