This study is to examine the change of fasting insulin, glucose, insulin sensitivity and related traits in response to the 14 wk treatment of omega-3 fatty acids, including fish oil (n=25) and flaxseed oil(n=25), in Chinese gestational diabetic patients. Corn oil(n=25), rich in omega-6 fatty acids, will be selected as a controlled oil. The intervention will start from the third trimester of these pregnant women and finish at the 4wk after the birth of their children. The investigators hypothesize that omega-3 fatty acids could improve insulin sensitivity and glucose metabolism in these patients.

Inclusion Criteria:

• 1)gestational diabetes 2) female, <40 years old 3)willing to participant in the trial

Exclusion Criteria:

• 1）type 1 diabetes or type 2 diabetes before pregnancy 2）Deny to sign the informed consent 3）Family history of hypertriglyceridemia or fasting triglyceride>4.56 mmol/L 4）Have severe liver disease, kidney disease or cancer 5)Participating in the other clinical trial within 30 days 6)Other diseases or conditions, for which the doctor of the patients do not agree his or her participating