The upregulation of Th1 cells has been suggested to have an essential function in the development of atherosclerosis (AS). Recent studies indicate that miR-146a is a microRNA specifically and highly expressed in Th1-driven autoimmune disease. The aim of the study was to investigate the possible mechanisms of the miR-146a in the onset of acute coronary syndrome (ACS). The results showed that the expression of miR-146a in peripheral blood mononuclear cells (PBMCs) was significantly increased in patients with ACS. We showed that overexpression of miR-146a in PBMCs could significantly upregulate the function of Th1 cells. Furthermore, we showed that miR-146a treatment could modulate the Th1 differentiation through posttranscriptional enhancing the T-bet pathway in PBMCs. In addition, this study also provided evidence that miR-146a treatment in vitro could induce the protein expression of TNF-alpha, MCP-1, NF-kappaB p65, which are key pro-inflammatory cytokines and critical transcription factor in AS. In contrast, miR-146a inhibitor could attenuate these phenomena significantly. The results support the concept that miR-146a may be a novel regulatory factor in Th1 differentiation and a new therapeutic target for AS and ACS.