In this study, patients who suffered from gestational diabetes in the academic outpatient clinic at Alzahra Hospital of the Tabriz University of Medical Sciences, will be assessed in order to determine the Effects of vitamin D supplementation on serum vitamin D levels, markers of metabolic and inflammation in women with gestational diabetes. Inclusion criteria consists of: gestational diabetes; age between 15 and 45 years old and gestational age 24-28 weeks and exclusion criteria are: thyroid and parathyroid disorders, kidney disease, heart disease, hypertension, smoking and usage of nutritional supplements containing vitamin D in the past 6 months. 45 subjects will be divided into two groups randomly (taking the vitamin D supplement and control). Participants will receive 4 oral treatments consisting of 50000 IU of vitamin D3 (D-Vit 50 000; Zahravi Pharm Co, Tabriz, Iran), or a placebo (Zahravi Pharm Co), 1 every 2 weeks for 8 weeks. Every 2 weeks, vitamin D and placebo will be delivered to people. For each Patient questionnaire will be filled and general characteristics, anthropometric measurements (height and weight to calculate BMI) will be assessed. For each patient, 24-h recall questionnaire will be filled in order to assessment of energy, macronutrient and vitamin D intake for 3 days a week (2 working days and 1 holiday). These questionnaires will be completed at baseline and 8th week of the study. Ten ml blood sample from each patient will be taken after 12-14 hours fasting at the beginning and end of the intervention, and serum 25 OH vitamin D, fasting blood glucose levels, HbA1c, fasting serum insulin, insulin resistance, serum lipids and hs-CRP in both groups before and after the intervention will be measured.