The purpose of this study is to explore the differences in efficacy and safety of sitagliptin,vildagliptin and saxagliptin and to find which one is more better in treating type 2 diabetes mellitus.

Inclusion Criteria:

• signed the informed consent
• diagnosed with T2DM
• women of childbearing potential were required to have a negative urine; pregnancy test,and agreed to use adequate contraception throughout the; study and for up to 4 weeks after completion
• glycosylated hemoglobin ranged in 6.5-9.5%

Exclusion Criteria:

• patients in pregnancy or lactation period
• ever received any kind of oral Hypoglycemic drug in recent 3 months
• patients were currently receiving treatment with a cytochrome P450 3A4 inducer or depressor,a systemic corticosteroid,or a human immunodeficiency virus anti-viral medication
• gastrointestinal surgery that could affect drug absorption
• patients with haemoglobinopathy or rapidly progressing renal disease, or autoimmune skin disorder
• a recent history of alcohol or drug abuse within the past 12 months
• any contraindication listed in the package inserts of the study drugs
• a history of acute or chronic Pancreatitis or currently
• type 1 diabetes; a history of diabetic ketoacidosis or hyperosmolar nonketonic coma
• New York Heart Association class III or IV congestive heart failure; left ventricular ejection fraction ≤ 40%; a major cardiovascular event within the past 6 months
• significant abnormal liver function, defined as aspartate aminotransferase or alanine aminotransferase > 2 times the upper limit of normal or total bilirubin >34 µmol/L (> 2 mg/dL); or a history of positive serologic evidence of infectious liver disease
• clinical signs or symptoms of active liver disease and/or significant abnormal liver function
• patients with any clinically significant abnormality identified on physical examination, electrocardiogram (ECG), or laboratory tests that, in the judgment of the investigator, would compromise the patients' safety or successful participation in the clinical study
• fasting plasma glucose level > 13.3mmol/l
• creatinine clearance rate ≤ 90ml/min