Abstract: Objective To investigate the clinical characteristics, metabolic status, insulin resistance and insulin secretory function of diabetic patients with early onset. Methods The study was undertaken in the West China Hospital of Sichuan University. Characteristics of 342 admitted diabetic patients with early onset (EOD group, diagnosed at age 15-45 years old) were reviewed and compared with 296 admitted patients with late onset (LOD group, diagnosed at age＞45 years old). All of the participants had negative islet autoantibodies. Homeostasis model assessment 2 of insulin resistant (HOMA2-IR) and HOMA2 of insulin sencitivity (HOMA2-%S) were measured to estimate insulin resistance and insulin sensitivity. HOMA2 of β-cell function (HOMA2-%β) index was used to estimate β-cell secretory function. We also compared clinical characteristics and metabolic status between the two groups. Results EOD patients were more likely to have ketosis,ketoacidosis, insulin therapy and positive diabetic family history than LOD patients (P<0.05). Levels of systolic blood pressure (SBP), diastolic blood pressure (DBP), body mass index (BMI), waist-to-hip ratio (WHR), fasting and postprandial insulin (FIns, PIns), fasting and postprandial plasma C-peptide (FCP, PCP) were significantly lower, and glycosylated hemoglobin A1c (HbA1c), triglycerides (TG), fasting and postprandial blood glucose (FPG, PPG) were significantly higher in EOD patients than in LOD patients (P<0.05). EOD patients had lower prevalence of hypertension, central obesity, hyperuricemia, metabolic syndrome (MS) and co-exist of three or more metabolic disorders than LOD patients (P<0.05). EOD patients had decreased levels of HOMA2-%β, ΔI30/ΔG30 and HOMA2-IR, increased HOMA2-%S, and increased proportions with FCP<0.2 nmol/L and FIns＜2.9 μU/mL CM(155.3mmcompared with LOD patient (P<0.05). Linear regression analyses showed that HOMA2-%β, ΔI30/ΔG30, positive diabetic family history were independent risk factors predicting early onset of diabetes. Conclusion Early onset diabetic patients are characterized with low prevalence of metabolic disorders, insulin resistance and severe insulin secretion dysfunction. Loss of β-cell function may play a major role in the development of early onset diabetes in this population.