Abstract: Objective To investigate body weight loss and influence factors during intensity-modulated radiotherapy (IMRT) in patients with nasopharyngeal carcinoma (NPC). Methods To explore body weight and BMI variation and influence factors，body weight and body mass index (BMI) change analysis were done retrospectively for 57 NPC patients during IMRT. Results In 57 NPC patients treated by IMRT, the pre-radiotherapy and post-radiotherapy average body weight were （60.4±9.4）kg and （55.1±8.7）kg (P=0.0025), body weight loss ranged from 17 kg to-0.5 kg，and average BMI of pre-radiotherapy and post-radiotherapy were （22.8±3.5）kg/m²，（20.8±3.1）kg/m² (P=0.0017), ranged from 6.0 kg/m² to -0.2 kg/m²，respectively. To the end of radiotherapy，93.0% patients lost weight at different levels, the mean percentage of weight loss was 8.4% and ranged from 23.4%to -0.7%, 35.1% patients lost weight over 10% and 68.4% patients lost weight over 5.0%. Univariate multi-factor analysis of variance showed that the WHO pathological grade, AJCC6th stage, performance status (PS), concurrent chemotherapy and late course accelerated hyperfractionated radiotherapy (LCAFR) had no statistical influence on body weight and BMI loss, but patients with age older than 50 years or BMI greater than 25 kg/m² or body weight more than 60 kg lost body weight and BMI more than others (P<0.05). Linear regression analysis showed that average radiation dose weekly and the average body weight weekly were significantly negatively correlated (R2=-0.996，P<0.001). Conclusion Body weight loss is widespread in NPC patients during IMRT. The age, pre-radiotherapy body weight and BMI, radiation dose maybe have more impact on body weight and BMI loss significantly rather than the gender，WHO pathological grade, AJCC6thstage, performance status, concurrent chemotherapy and late course accelerated hyperfractionated radiotherapy in NPC patients with IMRT.