The aim of this study is to assess the oxidative stress status in diabetes mellitus (DM) and diabetic nephropathy. The study group comprised 40 control subjects, 40 type 2 DM patients without complications and 37 diabetic nephropathies. Compared with control subjects, superoxide dismutase, glutathione peroxidase, catalase, vitamin C were decreased (P < 0.01). There was a significant increase in serum malondialdehyde (MDA), conjugated diene (CD), advanced oxidation protein products (AOPP), protein carbonyl (PC) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) in diabetes patients when compared with normal subjects (P < 0.01). Moreover, these indexes were much higher in diabetic nephropathy than that of diabetic patients without vascular complications (P < 0.05, P < 0.01). There was a significant correlation between the serum glucose levels and PC, 8-OHdG (P < 0.05, P < 0.01). There were highly significant positive correlation of CD and MDA, AOPP and PC (P < 0.01). Plasma AOPP levels had a significant correlation with PC levels (P < 0.01). Our findings suggested that diabetes patients have more severe oxidative stress than normal persons and higher oxidative stress in diabetic nephropathy than those in patients without complications. Oxidative stress may play an important intermediary role in the pathogenesis of diabetes complications.