【Abstract】 Objective The aim of this study was to compare IMRT optimization for Nasopharyngeal carcinoma in the CMS XIO radiotherapy treatment planning system, with and without segment weight optimization. Methods 30 patients with pathologically confirmed nasopharyngeal carcinoma were enrolled. Two IMRT plans were made(S-IMRT and SWO-IMRT) for each case, and which had the same gantry directions（153,102,51,0,309,258,207）and optimization parameters. Firstly, S-IMRT plans were optimized with the way of the static intensitymodulated method (step & shoot), transformed the intensity distribution into MLC sequence by the CMS XIO 464 planning system. Then, SWO-IMRT plans were generated based on S-IMRT plans using segment weight optimization tool (SWO). Comparing the difference of the number of segments, MU between the S-IMRT and SWO-IMRT plans. By the dosevolume histogram(DVH), and the target homogeneity index (HI),conformal index(CI), dose distribution in the volume of targets and OARs were analyzed. Results For the target areas (PGTVnx, PGTVnd, PCTV1 and PCTV2) D98, D95, D50 and conformal index (CI) in SWO-IMRT group were superior to S-IMRT group(P＜0.05), while no statistically significant differences were found in D2, homogeneity index (HI), PCTVnd and the OAR. The study showed a statistically significant reduction in the total number of segments (mean: 17%) with SWO-IMRT as compared to S-IMRT (P=0.000). And slightly reduction of in the total MU was observed with SWO-IMRT (P＞0.05). Conclusion This study shows that the segment weight optimization method significantly improve the target areas dose and reduce the total number of segments for IMRT nasopharyngeal carcinoma, thus reducing machine wear and patient treatment time.