The present study aims to investigate the association between functional health literacy in diabetes mellitus and HbA1c.   This study employed a cross-sectional analytical study by using the interview from the totaled 544 Thai patients with T2DM who were followed up at 3 public hospitals under the Ministry of public health, Thailand, and HbA1c was collected via medical records. Multiple linear regression was used to investigate the association between functional health literacy in diabetes mellitus and HbA1c. Among the total of 544 T2DM patients, 81.0% had adequate functional health literacy in diabetes mellitus and 73.0 % had HbA1c level ≥ 7 mg %. After adjusting for covariates, we found that adequate functional health literacy in diabetes mellitus was the significantly association with HbA1c (adjusted mean difference = -0.63; 95%CI = -1.04 to -0.23; p= 0.010). In terms of other covariates, age, body mass index, health insurance scheme, and duration of diabetes were also strongly associated with HbA1c.   Functional health literacy in diabetes and age, body mass index, health insurance scheme, and duration of diabetes are the most importance factors associated with HbA1c in patients with T2DM. Therefore, the public health intervention should be addressing functional health literacy in diabetes to improve glycemic outcomes in T2DM patients.