Gestational diabetes mellitus (GDM) is associated with not only adverse outcomes in pregnancy, such as preeclampsia, Cesarean delivery, macrosomia, and birth trauma but also increased risk of conversion to type 2 diabetes after delivery. Nowadays, the prevalence of GDM has been increasing in the world, especially in obese women. Although treatment of women with GDM is very important and well-known, a better way is prediction and prevention of GDM in obese women. In this talk, I will present how to predict and prevent GDM in obese women.