Diabetes mellitus is one of the most common metabolic and chronic diseases of school age children. The main aim of diabetes management for diabetic teenage is to achieve optimal glycymic control. Peer support may help achievement of this aim. Aim: is to investigate the effect of nursing intervention on the supportive role played by peers on glycymic control among diabetic school students in Tanta city. Research design: Quasi-experimental research design. Material and method: The study was conducted at the outpatient medical clinics of all school health units (I, II, and III) affiliated to the students' health insurance serving Tanta city as well as governmental schools. A convenient sample of diabetic school children were selected from the previous settings representing fifty percent of the total subjects. The study sample was divided into two equal groups (control& study). The control group received an individual program for glycemic control. The study group received the same glycemic control program in addition to peer support sessions. An interview questionnaire and anthropometric measurements sheets were used to collect data and for evaluation. They were used three times during the study period; before the program, immediate, and three months post program. Results: the result of this study showed that before implementation of the program the study and control groups showed poor glycemic control, low scores of knowledge, self care practices and perceived peer support. After implementation of the program, the two groups showed significant glycemic control and improvement in their knowledge and self care practices. This improvement was highly significant among the study group who received peer support than the control one.Conclusion and recommendation: peer support approach is a predictor of glycemic control among school age children. Therefore, the school health nurse could do her best effort to make peer support available to every diabetic student, in particular, those with poor glycemic control or newly diagnosed.