Objective： The circadian rhythm of DNA synthesis in nasopharyngeal carcinoma (NPC) cells was investigated so that to provide necessary data for making clinical chrono-chemotherapy schedule of NPC. Methods： Fifteen BALB/c mice were synchronized with an alternative lighting regimen with 12 hours for light and 12 hours for dark (12∶ 12 LD) at least 3 weeks. Human nasopharyngeal poorly differentiated squamous cell carcinoma(CNE-2) cells were implanted into both flanks of each mouse. Ten days after transplantation, sampling from tumor was conducted in the 3, 9, 15, 21 hours after light onset (HALO). Single cell suspension was obtained and stained with propidium iodide. The cellular DNA content was measured by flow cytometry. Data were documented by ANOVA and Cosinor analysis. Results： The proportion of tumor cells in Phage G1,S,G2-M varied according to circadian time with statistical significance. The distribution curves of Phage G1 and Phage G2-M fit for Cosinor changes. Conclusion： DNA synthesis of human nasopharyngeal poorly differentiated squamous cell carcinoma(CNE-2) cells trausplanted in nude mice varies according to circadian rhythm.