Background: Gestational diabetes mellitus (GDM) is a worldwide problem affecting about 16.5% of pregnancies all over the world, with serious maternal and fetal complications. Vitamin D (Vit D) deficiency during gestation is widespread and during the last 10 years. A lot of studies found an association between decreased Vit D and impaired glycaemia concluding that deficient Vit D might be a modifiable risk factor for GDM. Objective: The aim was to detect whether maternal Vit D insufficiency during pregnancy is a risk factor for GDM. Materials and Methods: Case-control study done in Outpatient Clinics at Zagazig University Hospitals to assess relation between GDM and low Vit D. 96 pregnant women were studied, 48 had GDM (diagnosed by OGTT according to ADA criteria) and the other 48 had normal pregnancy. 25-Hydroxyvitamin D (S.25 OHD) was measured. Results: Increased prevalence (68.75%) of Vit D insufficiency was found in both groups. S.25 (OH) D was significantly lower in ladies with GDM in comparison with controls (26.8 ± 32.2 ng/ml vs 42.6 ± 4.2 ng/ml). Fasting blood sugar in early gestation has insignificant negative correlation with 25 (OH) D level (r = -0.06) Conclusion: There is statistically significant difference between case and control groups in 25 (OH) VIT D level with higher occurrence of Vit D deficiency/insufficiency in the GDM group