This is a Phase 1, open-label, sequential, multiple-dose, drug-drug interaction study of GK activator HMS5552 and metformin in patients with type 2 diabetes mellitus (T2DM). The study is to assess the potential pharmacokinetic interaction between HMS5552 and metformin in subjects with T2DM. The study is also to evaluate the safety and tolerability of HMS5552 with simultaneous administration of metformin in subjects with T2DM.