Objective: to evaluate serum 25-hydroxyvitamin D(25OHD) level in pregnant women who developed gestational diabetes mellitus(GDM)in comparison to healthy pregnant women, and to investigate the relationship between maternal hypovitaminosis D and insulin resistance, parathyroid hormone, and perinatal outcome. Methods: One hundred and sixty pregnant women between 24-28 weeks gestation were recruited. Group (A) 80 pregnant women with established diagnosis of GDM, and Group (B) 80 healthy pregnant women as control.GDM was diagnosed by 75 gm oral glucose tolerance test, and insulin resistance was measured by homeostasis model assessment of IR (HOMA-IR).25-hydroxyvitamin D, and parathyroid hormone (PTH) were measured in all participants. Results: 116 (72.5%) of participants had 25OHD deficiency. The mean serum 25OHD level was significantly lower in GDM group compared to control (29.7±14.6 vs. 48.4±22.5 respectively).Serum 25OHD levels showed significant negative correlation with BMI, blood glucose, HOMA-IR and PTH. Conclusions: Vitamin D deficiency is a prevalent problem among the pregnant women and can be considered as potential risk factor for development of GDM with subsequent deleterious effects on maternal and neonatal outcome. Keywords: Hypovitaminosis D, Gestational diabetes mellitus, Insulin resistance, Parathyroid hormone.