Objective：To explore the best time of using vitamin C to protect salivary gland radiation injury in patients with differentiat－ed thyroid cancer（DTC） after 131I therapy. Methods：Totally 120 DTC patients underwent thyroidectomy were randomly divided into groups A and B（n=60）. Patients were administrated with vitamin C in either 2 h or 24 h after the treatment of 131I in order to reduce damage caused by radioactivity of 131I. Salivary glands dynamic images were taken six months later and time-activity curves were drawn；then parotid and submandibular region of interest（ROI） and appropriate organization as background correction were lain out；finally，gland uptakes ratios（UR） and excretory index at maximum secretion（MSR） were calculated. Results：UR and MSR of left parotid gland were 3.16 ± 1.12，（61.13 ± 13.85）% in group A and 2.84 ± 1.10，（58.56 ± 12.26）% in group B；UR and MSR of right parotid gland were 3.08 ± 1.02，（63.70 ± 14.12）% in group A and 2.82 ± 0.95，（59.40 ± 12.27）% in group B；UR and MSR of left submandibular gland are 2.10 ± 1.01，（47.52 ± 12.74）% in group A and 2.15 ± 0.97，（49.92 ± 12.93）%，in group B；UR and MSR of right submandibular gland were 2.21 ± 1.05，（49.98 ± 13.20）% in group A and 2.15 ± 0.97，（49.37 ± 13.04）% in group B. Differ－ences of results in groups A and B were not statistically significant（P >0.05）. Conclusion：Salivary stimulation with vitamin C in both 2 h and 24 h after131I administration has no effect on salivary protection in DTC patients.