To investigate the effect and mechanism of miR-498 on Th17 cell differentiation of peripheral blood mononuclear cells (PMBCs) in rheumatoid arthritis (RA) patients, peripheral blood samples were collected from RA patients and healthy controls, respectively. The proportion of CD4+IL-17+ T cells (Th17 cells) or CD4+FOXP3+ T cells (Tregs) in T cells and the Th17/Treg ratio were identified by the flow cytometer. The STAT3 and miR-498 expression were measured by Western blot and real-time PCR, respectively. ELISA was used to detect IL-17 concentrations. Luciferase assay was performed to confirm that miR-498 directly targeted the 3' untranslated region (3'UTR) of STAT3 in CD4+ T cells. The effect of miR-498 on Th17 cell differentiation was explored by transfection of miR-498 mimic and/or pcDNA-STAT3 into CD4+ T cells. In PMBCs of RA patients, the Th17/CD4+ T cell ratio was significantly increased, while the Tregs/CD4+ T cell ratio was obviously decreased, leading to a higher Th17/Treg ratio. The results showed a reduced miR-498 expression and an increased STAT3 protein expression in PMBCs, and an increased IL-17 concentration in serum of RA patients. In cells transfected with wild-type-STAT3-LU, miR-498 mimic significantly reduced the luciferase activity, STAT3 gene and protein expression, and miR-498 inhibitor had an opposite function. While the miR-498 mimic/inhibitor had no effect on the luciferase activity and STAT3 expression in cells transfected with mutant-STAT3-LU. CD4+ T cells transfected with miR-498 mimic had a lower Th17/CD4+ T cell ratio and IL-17 concentration, however, transfection of pcDNA-STAT3 reversed the effect of miR-498 mimic on Th17/CD4+ T cell ratio and IL-17 concentration. These results suggest that overexpression of miR-498 suppresses Th17 cell differentiation by targeting STAT3 in RA patients.