Objective: To study cell cycle retardation, apoptosis and the expression of antioncogene p57kip2 by radioactive rays in nasopharyngeal carcinoma cells. Methods: Cell cycle retardation, apoptosis and cell survival rate induced by radioac- tive rays were tested by the methods of flow cytometry and MTT method. The expression of antioncogene p57kip2 was detected by immunohistochemistry and Western blot. Results: After irradiation, G1 phase had no obvious retardation, S phase showed transient delay. There was a positive correlation between irradiation dosage and retardation strength in G2/M phase (P < 0.01). Peak value appeared at 24 h after 12 Gy irradiation, then decreased. There was a positive correlation between apop- tosis incidence and irradiation dosage or after-irradiation time extention (P < 0.01). There was a negative correlation between cell survival rate and irradiation dosage or apoptosis incidence (P < 0.01). The expression of p57kip2 protein was up-regulated along with the prolongation of time and dosage after irradiation (P < 0.01). Conclusion: G2/M phase arrest, apoptosis and the up-regulation of the expression of p57kip2 protein all can reflect predict the radiosensitivity of nasopharyngeal carcinoma cells. Objective: To study cell cycle retardation, apoptosis and the expression of antioncogene p57kip2 by radioactive rays in nasopharyngeal carcinoma cells. Methods: Cell cycle retardation, apoptosis and cell survival rate induced by radioac- tive rays were tested by the methods of flow cytometry and MTT method. The expression of antioncogene p57kip2 was detected by immunohistochemistry and Western blot. Results: After irradiation, G1 phase had no obvious retardation, S phase showed transient delay. There was a positive correlation between irradiation dosage and retardation strength in G2 / M phase ( There was a negative correlation between apop- tosis incidence and irradiation dosage or after-irradiation time extention (P <0.01). There was a negative correlation between cell survival rate and irradiation dosage or apoptosis incidence (P <0.01). The expression of p57kip2 protein was up-regulated along with the pr olongation of time and dosage after irradiation (P <0.01). Conclusion: G2 / M phase arrest, apoptosis and the up-regulation of the expression of p57kip2 protein all can reflect predict the radiosensitivity of nasopharyngeal carcinoma cells.