INTRODUCTION: Gestational diabetes mellitus (GDM) is defined as a glucose intolerance that first occurs during pregnancy (1). ABO blood group has an important role in the human body Physiology.Early detection and treatment of GDM reduce its complications (3).So far, the relationship between blood group and 70 disease has been identified.Therefore, the present study were conducted,for determination the relationship between ABO blood groups with GDM. METHODS: this cross sectional study conducted between December 2011 and January 2012,and samples were selected from 1262 pregnant women admitted to hospitals and health centers in Gorgan .For all patients, blood group tests and glucose challenge test(GCT) with 50 g glucose was done. And for those participent who have positive GCT, The oral glucose tolerance test (OGTT) were performed. The obtained data By using, SPSS software and statistical test chi-scquare were analyzed at a significant level of p≤0.05. RESULTS: In non o blood group, 46 (6%) of participent have GDM while, only 12(2.8%) of women with O blood group had GDM that was assosiated with statistically significant difference. Also among non o Blood group, AB blood group with 7.4% was associated with the highest risk of GDM. After that, there were ordinally, blood group B with 6.9%, and blood group A,with 4.7%. that was assosiated with statistically significant difference. CONCLUSION: The findings showed that blood group O have preventive effects on the development of GDM and subjects with non O blood group are more likely to suffer from GDM.