Objective To examine the application of On-Board Imaging (OBI) system-based image-guided radiotherapy (IGRT) in the improvement of the precision of intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma.Methods Ten patients with nasopharyngeal carcinoma were treated with IMRT using the OBI system. The IGRT images after positioning, position adjustment, and treatment were observed and recorded to investigate the image difference between CT simulation and IGRT. Results The deviations in the x (lateral), y (cranial-caudal), and z (ventral-dorsal) directions between CT simulation and IGRT images were 0.22±1.00 mm,-0.37±1.28 mm, and 0.04±1.36 mm, respectively, after positioning, 0.29±0.76 mm,-0.04±0.78 mm, and -0.01±0.92 mm, respectively, after position adjustment, and 0.20±0.78 mm, 0.16±0.80 mm, and 0.05±0.92 mm, respectively, after treatment. The probabilities of a ≤1 mm deviation in the x, y, and z directions were 81.0%, 77.6%, and 88.2%, respectively, after positioning, 92.5%, 96.4%, and 96.4%, respectively, after position adjustment, and 91.7%, 94.9%, and 96.8%, respectively, after treatment. Conclusions The application of OBI system-based IGRT is very important in the improvement of the precision of fractionated IMRT for patients with nasopharyngeal carcinoma. The position of the patient should be adjusted based on the IGRT image after positioning in order to correct set-up error and effectively increase the precision of fractionated IMRT.