Background: Vitamin D is part of many physiological functions in the body. This is enough reason to optimise vitamin D level in the body. Studies have linked vitamin D deficiency to the development of Gestational Diabetes Mellitus (GDM). We aimed to compare Vitamin D levels in GDM and Non GDM pregnant women at King Abdulaziz University Hospital Jeddah, Kingdom of Saudi Arabia. Methods: This cross-sectional study was conducted at Obstetrics and Gynaecology outpatient clinic and inpatient obstetrics ward at King Abdulaziz University hospital, Jeddah, Saudi Arabia from January to April 2016. 92 subjects who fulfilled the inclusion criteria were enrolled in the study. Results: Vitamin D level in both the GDM & non GDM groups showed no statistically significant difference. All the 92 female patients were distributed into four quartiles depending on vitamin D level. ,30.4% of quartile 4 was GDM while 47.8 % of quartile 1 was GDM with no statistical significance. The mean age in the GDM group was 31.9?4.8 and in the non GDM was 26.9?5.6 which was statistically significant (p=0.001). The mean score of BMI in GDM group was 32.3?5.8 and non GDM was 28.3?5.5 which was statistically significant (p=0.002). No statistically significant difference was found among the two groups as per the following parameters: Gestational Age (GA), gravity & parity. Conclusion: We found no statistical difference between the GDM and Non GDM groups as regards Vitamin D. Subjects with higher BMI and higher age had significantly higher incidence of GDM. When the results were arranged according to quartiles with lowest vitamin D in quartile 1 and highest vitamin D in quartile 4, we found that the number of GDM cases in the first quartile were double (N=12-47.8%) than the number of GDM cases in the fourth quartile (N=6-30.4%) this difference did not show a statistical significance.