Background: There are some explanations for the relationship between vitamin D (Vit. D) low levels and type 1 diabetes mellitus (T1DM). T1DM is an autoimmune disorder that affects pancreatic cells, Vit. D acts an immunomodulatory function via the Vit. D receptor abundant on immune and pancreatic cells. By lowering the number of effector "T" cells, it has been demonstrated that great concentrations of 1,25-dihydroxyvitamin D positively lower the diabetes' occurrence. Our study's aimed to assess the potential role of Vit. D concentration in early infancy as an indicator of possibly developing islet autoimmunity or clinical T1DM. Method: This was a case-control study involving 40 T1DM-diagnosed children aged 6 to 12 years and 20 healthy children as controls with comparable gender, height (cm), age, weight (Kg), and body mass index (BMI). For all subjects, serum hydroxyl Vit. D (25 OHD) was measured using chemiluminescence immunoassay. Serum total Ca, ph, ALP and PTH were measured. Also, HbA1C% was measured win both groups, and blood glucose was also measured. Mg, AST and ALT were also assessed. Results: The mean serum 25OHD in T1DM children was less than Healthy controls 9.3+1.4Vs 59.2+18.7ng/ml respectively with (p < 0.0001) and diabetic cases showed negatively significant relationship between vitamin D versus HbA1c% and glucose, (r = -0.8, -0.9) respectively (p < 0.0001). Conclusion: Vitamin D deficiency is frequent in children with T1DM, and it is important to pay attention to this issue. Future studies will be interested in investigating vitamin D supplementation for autoimmune diseases, particularly T1DM.