We measured levels of fasting blood glucose (FPG), 2-hour postprandial blood glucose (2hPG) and HbAlc for 91 patients of DM with complications so as to study the diagnostic and prognostic significances of glycosylated hemoglobin (HbAlc) in diabetes mellitus (DM). With regression analysis the HbAlc concentration frequency distribution in DM was assessed. Our results indicated that the HbAlc concentration at frequency distribution P50 as measured in DM is 5.94%. HbAlc is correlated with FPG and 2hPG, with correlation coefficient r1=0.48998 and r2= 0.29995, respectively. The relevance analysis of HbAlc concentration and DM complication shows that the higher the HbAlc content is, the higher the incidence rate of DM complication will be. Therefore, HbAlc shows a high specificity for DM diagnosis. It is also implicated in the prediction of DM complication and the adjustment of therapeutic schedule.