Background and Objective: Anemia can not only reduce the quality of life of patients with cancer, but also affect their survival. This study was to investigate the prognostic value of hemoglobin (Hb) level in patients with nasopharyngeal carcinoma (NPC) treated with radiotherapy. Methods: Clinical data of 520 NPC patients received definitive radiotherapy between 2000 and 2002 at Sun Yat-sen University Cancer Center were analyzed. Patients were stratified into normal Hb level and anemia groups according to their Hb levels before, during, and after radiation. Anemia was defined according to World Health Organization criteria as Hb level < 130 g/L in men and < 120 g/L in women. Hb continuous decrease group and non-decrease group were defined according to Hb changes in the patients during radiotherapy. Loco-regional recurrence-free survival (LRFS) and overall survival (OS) rates were estimated using the Kaplan-Meier method. Multivariate analysis was performed using the Cox model to analyze the prognostic factors. Results: Before radiation, the 5-year LRFS rates were 60.9% in anemia group and 63.9% in normal Hb level group (P = 0.337); the 5-year OS rates were 65.2% and 71.0%, respectively (P = 0.299). During radiation, the 5-year LRFS rates were 56.7% in anemia group and 67.9% in normal Hb level group (P = 0.013); the 5-year OS rates were 61.0% and 75.9%, respectively (P = 0.001). After radiation, the 5-year LRFS rates were 59.6% in anemia group and 64.9% in normal Hb level group (P = 0.169); the 5-year OS rates were 65.0% and 71.9%, respectively (P = 0.090). The 5-year LRFS and OS rates were significantly lower in Hb continuous decrease group than in Hb non-decrease group (59.1% vs. 69.3%, P = 0.032; 66.2% vs. 76.4%, P=0.011). Multivariate analysis showed that the continuous decrease of Hb was an independent prognostic factor for OS. Conclusion: The change in Hb level during radiotherapy is an important prognostic factor affecting the OS of NPC patients.