Vitamin D (Vit D) deficiencymay be linked to the development of type 1 and type 2 diabetes mellitus (DM). Specific Vitamin D receptor (VDR) allelic variants have been linked totype-1 diabetes mellitus (T1DM) in many countries. However, in type 2 DM, Vitamin D deficiency leads to impaired insulin action and secretion in both animal models and humans. This study aims to determine the effect of low vitamin D on glycemic control in Egyptian patients with type 1and type 2 DM. Patients and Methods: This is a retrospective multi-centers study included280 diabetic patients (140 patients with type 1 DM, and 140 with type 2 DM). The patients\' data were collected from the patients\' medical files in the Diabetic Clinics and Internal Medicines Departments of 3 University hospitals (Benha, Helwan, and Assiut) in Egypt, including age, sex, BMI, onset and duration of DM, LDL-C, vitamin D, and HbA1c. Results: 50 (35.7%) type 1 diabetic patients had low vitamin D versus 110(78.6%) type 2 diabetic patients. The study revealed asignificant inverse correlation between the serum vitamin D level and HbA1c levelin patients with type 1 DM; but,in patients with type 2 DM,there was no significance correlation between serum vitamin D level and HbA1c level. Conclusion: low serum vitamin D level correlated inversely with the degree glycemic control in type 1 DM; but,such correlation had been failed to prove in patients with type 2 DM.Whether giving vitamin D supplementation to already vitamin D deficient diabetics will help to modulate the disease progression remains a debate.