Objectives. To describe the characteristics of long-standing T1DM in Thai patients and assess residual beta-cell function with status of pancreatic autoantibodies. Methodology. This is a cross-sectional study of Thai subjects with T1DM and disease duration ≥25 years seen at the Theptarin Hospital. Random plasma C-peptide and pancreatic auto-antibodies (Anti-GAD, Anti-IA2, and Anti-ZnT8) were measured. Patients who developed complications were compared with those who remained free of complications. Results. A total of 20 patients (males 65%, mean age 49.4±12.0 years, BMI 22.5±3.1 kg/m2, A1C 7.9±1.6%) with diabetes duration of 31.9±5.1 years were studied. Half of the participants remained free from any diabetic complications while the proportions reporting retinopathy, nephropathy, and neuropathy were 40%, 30%, and 15%, respectively. HDL cholesterol was significantly higher and triglyceride concentration significantly lower in patients who were free from diabetic nephropathy but not in those who were free from other complications. The prevalence rates of anti-GAD, anti-IA2, and anti-ZnT8 were 65%, 20%, and 10%, respectively. None of the patients who tested negative for both anti-GAD and anti-IA2 was positive for anti-ZnT8. Residual beta-cell function based on detectable random plasma C-peptide (≥0.1 ng/mL) and MMTT was found in only 3 patients (15%). There was no relationship between residual beta-cell function and protective effects of diabetic complications. Conclusion. Endogenous insulin secretion persists in some patients with long-standing T1DM and half of longstanding T1DM in Thai patients showed no diabetic complications. HDL cholesterol was significantly higher and triglyceride concentration significantly lower in patients who were free from diabetic nephropathy.