Abstract： Objective To investigate the effect of rosiglitazone and insulin on islet β cell function in latent autoimmune diabetes in adults (LADA). Methods Glutamic acid decarboxylase (GAD) antibody was screened in patients initially diagnosed type 2 diabetes. LADA patients, defined as positive GAD antibody and with a fasting C peptide of 300 pmol/L or more, were selected and randomly assigned to receive subcutaneous insulin alone ( n =6) or rosiglitazone combined with insulin ( n =6) to compare the changes of islet β cell function. At entry and 1 year after treatment, blood was drawn to determine plasma glucose, HbA 1c and C peptide at fasting and 2 hours after taking 75 g glucose without medication. GAD antibody and C peptide were measured with radioligand immunoassay and radioimmune assay respectively. Results After 1 year treatment, LADA patients in rosiglitazone and insulin group had a higher postprandial C peptide than those in subcutaneous insulin group(782.1 pmol/L vs 1 695.0 pmol/L, P <0.05). During 1 year observation, C peptide levels in patients treated with insulin alone had a tendency to decline (from 1 710.5 pmol/L to 782.1 pmol/L) while that of the rosiglitazone combined with insulin group stayed steady (from 1 242.9 pmol/L to 1 695.0 pmol/L). The difference of postprandial C peptide before and after 1 year treatment(post treatment subtract pre treatment) was significantly bigger in combined treated group than that of insulin alone group (243.0 pmol/L vs 1 079.6 pmol/L, P <0.05). The linear stepwise regression analysis showed that postprandial C peptide level and body mass index (BMI) at entry, different treatment regime were positively correlated with postprandial C peptide after 1 year, of which the last factor played a key role. No severe side effects were observed in both groups. Conclusion Rosiglitazone combined with insulin may help preserve β cell function in LADA patients.