Abstract： Objective To investigate the clinical characteristics of the familial early onset type 2 diabetes（T2DM）,and the factors affecting the age at diagnosis. Methods 190 probands from early onset T2DM pedigrees （diagnosed before 40 years old） and 103 probands from late onset T2DM （pedigrees） （all patients were diagnosed after 40 years old）were included in this study. The homeostasis model assessment of insulin resistance （HOMA-IR）was used to estimate insulin resistance, HOMA-β cell was used to evaluate basal insulin secretion. The ratio of the incremental insulin (ΔI30) over （incremental） glucose(ΔG30) was calculated after intake of a steamed bread made of 100g flour to （evaluate） the early insulin secretion. Body mass index（BMI）and serum lipid levels were also （determined.） （Results） （1）The early onset group had significantly higher fasting serum insulin （（natural） logarithm transformed, 2.6±0.6 vs 2.4±0.5 μU/ml, P<0.05）, triglycerid （TG, 1.7±2.0 vs 1.4±2.0 mmol/L,P<0.05）,HOMA-IR（natural logarithm transformed, 1.6±0.8 vs 1.3±0.6, P<0.01）and lower high density lipoprotein （HDL-C, 1.2±0.3 vs 1.3±0.3mmol/L,P<0.05） than those in late onset group.（2）Linear regression analyses showed that HOMA-IR, HOMA-β were （independent） risk factors predicting early age of onset. （3）In the early onset type 2 diabetic patients, the subgroup with BMI more than 25 kg/m2 had significant earlier diagnosis age （32±7 vs 34±5 y, P<0.05）, lower level of serum HDL-C （1.1±0.3 vs 1.3±0.3 mmol/L, P<0.01）, higher FIns （（natural） logarithm transformed, 2.8±0.6 vs 2.4±0.6, P<0.01）, higher HOMA-IR （natural （logarithm） transformed, 4.3±0.9 vs 3.9±1.0, P<0.01）and higher TG （1.9±2.0 vs 1.5±2.1 mmol/L, P<0.01）than those with BMI less than 25 kg/m2. Conclusion Compared with late onset familial type 2 diabetic patients,more severe insulin resistance and relative β cell function defect are the important clinical （features） in early onset familial type 2 diabetic patients. The interaction between （insulin resistance） and （β-cell） dysfunction are probably the determinants of onset age of T2DM.