Aim To investigate the effects of metformin on the levels of plasma total homocysteine(tHcy) in patients with type 2 diabetes mellitus(DM).Methods The levels of tHcy was determined by high performance liquid chromatography(HPLC) with fluorescence detection in 30 untreated type 2 DM cases and 30 healthy cases as control.30 DM patients were treated with metformin for three months(0.75～1.00 g/d),after the treatment,tHcy was detected again.Results tHcy was significantly higher in type 2 DM compared to controls(14.1±5.7 μmol/L vs 9.5±3.4 μmol/L,p<0.01),and was correlated with age,BMI,FBG,FINS(p<0.05),negative correlated with ISI(p<0.05).Following three months treatment with metformin,tHcy levels was significantly decreased(11.6±4.7 μmol/L,p<0.05).Conclusion The levels of tHcy increase in type 2 DM.After treating with metformin,the levels of tHcy decrease.