This study is a phase III study in subjects with T2DM. As designed, the study will start with a 4-week, single-blind, placebo run-in period based on diet and exercise interventions for screening eligible subjects, meanwhile, subjects are treated with Metformin (Glucophage) at 1500mg/day as basic therapy throughout the treatment period. 3-weeks after run-in, eligibility is confirmed with required laboratory tests at -5±2 days prior to randomization. The eligible subjects are randomly assigned to HMS5552 75mg BID group or placebo BID group with ratio 1:1 to receive a 24-week double-blind treatment. Then all subjects receive 28-week open-label treatment of HMS5552 75mg BID add-on to Metformin. After 52-week treatment, all investigational drugs should be discontinued and followed by 1-week for safety evaluation. The dosage of Metformin (Glucophage) should be maintained at 1500 mg/day from run-in period to treatment completion.