OBJECTIVE： To observe clinical efficacy and safety of sitagliptin combined with metformin in the treatment of type 2 diabetes complicating with metabolic syndrome. METHODS： Totally 80 patients with type 2 diabetes complicating with metabolic syndrome were divided into the observation group and control group with 40 cases in each group according to simple random sampling method. Both groups were given same diet and exercise plan； control group was additionally given metformin orally， 0.5 g each time， tid； observation group was additionally given sitagliptin， 100 mg each time， qd， on the basis of control group. Treatment course of 2 groups lasted for 12 weeks. The waist circumference （WC）， body mass index （BMI）， blood glucose， blood lipid， fasting insulin （FINS） and serum adiponectin were detected in 2 groups， and steady-state model insulin resistance index （HOMA-IR） and whole body insulin sensitivity index （WBISI） were calculated. The occurrence of ADR was observed. RESULTS： Before treatment， there were no statistically significant differences between 2 groups in WC， BMI， systolic blood pressure， diastolic blood pressure， triglycerides， cholesterol， high-density lipoprotein cholester， low-density lipoprotein cholesterol，fasting plasma glucose， 2 h postprandial blood glucose， FINS， HOMA-IR， WBISI and serum adiponectin （P＞0.05）. After treatment， the above indexes of 2 groups were obviously improved， and the improvement of observation group was significantly better than that of control group， with statistical significance （P＜0.05）. 2 cases and 3 cases were withdraw from the study in control group and observation group because of ADR， respectively.There was no statistically significant difference in the incidence of ADR between 2 groups （P＞0.05）. ?CONCLUSIONS： Sitagliptin combined with metformin shows significant clinical efficacy in the treatment of type 2 diabetes complicating with metabolic syndrome. Its mechanism may be related to reducing insulin resistance， enhancing insulin sensitivity， decreasing patients’ body weight and up-regulating serum adiponectin level.