Objective: We assessed the prevalence and pregnancy outcomes of gestational diabetes mellitus (GDM) in Dongguk University Ilsan hospital between August, 2005 and March, 2018. Methods: 4316 singleton and 722 twin pregnancies from 23 gestational weeks resulting in live births were included. The adjusted odds ratios(aOR) of adverse outcomes were evaluated comparing no diabetes and GDM pregnancies, controlling for maternal characteristics in singleton and twin pregnancies. Results: GDM complicated 6.0% of singleton pregnancies, and 7.0% of twin pregnancies. In singleton pregnancies, GDM were associated with increased risks in cesarean delivery (aOR 1.48, p<0.01),false positive of chromosome anomaly screening test (aOR 1.21, p=0.04), and large for gestational age (LGA) infants (aOR 1.87, p<0.01). In twin pregnancies, GDM was associated with increased risks of spontaneous preterm delivery (aOR 1.13, p=0.04), small for gestational age (SGA) infants (adjusted OR 1.23, p=0.02), and LGA infants (adjusted OR 1.73, p<0.01). Conclusion: GDM is associated with adverse outcomes in singleton and twin pregnancies, and the increased risks are generally similar or less in twins. Especially GDM is associated with fetal growth restriction in twin pregnancies.