Background. Studying effects of polarized light therapy on gingivitis in type I diabetic adolescents is a strategy for determining its efficacy in healing effect on gingival ulceration. Objective: to evaluate the efficacy of polarized light therapy on gingivitis in type I diabetic adolescents. Methods. Thirty type I diabetic adolescents receiving insulin therapy (Males and Females) who had gingivitis, ulceration pain and their ages ranged from 12 to 17 years were divided into two groups. Group (A) composed of 15 patients received the Bioptron light therapy (BLT) in addition to the routine medical care of gingivitis and insulin therapy. Group (B) received only the routine medical care of gingivitis and insulin therapy, duration of the BLT application was 10 minutes applied daily for 30 days. Results. Results showed that application of the BLT had a valuable healing effects on gingival ulceration and decrease pain in type I diabetic adolescents as evidenced by the highly decreases of the WHO oral mucositis scale, the common toxicity criteria scale and visual analogue scale. Conclusion. BLT is an effective additional tool for physical therapy program in treatment of gingivitis in type I diabetic adolescents as it plays an important role in healing effect on gingival ulceration and decrease pain.