Circulating endothelial progenitor cells (EPCs) play an important role in the development and progression of diabetic vascular complications. The aim of this study was to investigate the effects of gliclazide plus metformin (GLIMET) compared with metformin alone (MET) on number and function of circulating EPCs in T2DM patients. Patients with newly diagnosed T2DM were randomly divided into two groups, receiving the following treatments for 16 weeks: MET group (assuming metformin 500-2500 mg/day, n=24) and GLIMET group [assuming gliclazide (modified release, 30-60 mg/day)+metformin (250-1000 mg/day), n=23]. Circulating EPCs were quantified by flow cytometry, and the ability to uptake LDL and stain for lectin were used as another method of characterizing EPCs ex vivo. The functions of circulating EPCs were evaluated by colony-forming units (CFU) and migration. The status of oxidative stress was analyzed by serum-free malonaldehyde (MDA) and superoxide dismutase (SOD). There were no significant differences in clinical characteristics and number and function of circulating EPCs between two groups at baseline. Glycemic responses were similar after treatments. Compared with MET group, GLIMET group was associated with an increase in circulating EPCs number, DiLDL-lectin-positive EPCs, and migration. The mean improvements in MDA and SOD of GLIMET group were more strongly upregulated than those of MET group. This study demonstrated that both metformin mono-treatment and metformin plus gliclazide combination treatment provided with improvements in number and function of circulating EPCs. Compared with metformin mono-treatment, early use of combination therapy with gliclazide plus metformin made more effective improvements in circulating EPCs.