Background: Atherosclerosis (AS) is defined as chronic inflammation of the vessel wall. The major objective of the this study was to explore themechanism of Treg/Th17 imbalance and the role of high mobility group box-1 protein (HMGB1) on the balance in AS. Methods: We detected the apoptotic ratios of Treg and Th17 cells in peripheral blood mononuclear cells (PBMCs) from subjects with AS and normal coronary arteries (NCA) by flow cytometry. The effects of recombinant HMGB1 (rHMGB1) on the proportion, apoptosis and differentiation of Treg and Th17 cells were analyzed using flow cytometry, qRT-PCR and ELISA. Results: The frequencies of apoptotic Treg cells in the PBMCs from the subjects with AS were significantly higher than in those with NCA (p < 0.01). Stimulation of rHMGB1 obviously increased the level of Th17 cells and acidrelated orphan receptor C (RORC) mRNA, and markedly decreased Treg cell frequency and themRNA expression of factor forkhead family protein 3 (Foxp3) in the PBMCs. rHMGB1 played an obvious role in elevating Treg cell apoptosis ratio (p < 0.01). rHMGB1 treatment significantly decreased Treg cell ratio and IL-10 level, and increased Th17 cell ratio and IL-17A level induced from naïve CD4+ T cells. Conclusions: HMGB1 may modulate Treg/Th17 balance in patients with AS through inducing Treg cell apoptosis and promoting cell differentiation of Th17.