Abstract: M-xylene bicyclic peroxy radical (M-BPR) is an important atmospheric intermediate formed by the oxidation of m-xylene, which plays an important role in the new particle formation and growth of secondary organic aerosol. In this work, the reaction mechanism, thermodynamic analysis, and kinetics of the reaction between M-BPR and HO₂ were investigated at the CCSD(T)/cc-pVDZ//B3LYP/6-311G(d,p) level of theory coupled with transition state theory. The calculated results indicate that the title reaction can occur on both singlet and triplet potential energy surfaces, and the formation of hydroperoxides and ³O₂ via triplet state is the main reaction channel, while the other four singlet product channels are negligible due to the higher barrier heights. Additionally, the reaction rate constants are estimated by using the transition state theory over the temperature range of 258 K to 378 K, and reaction rate constants are found to be negatively correlated with temperature. At 298 K, the total rate constant for the title reaction is 1.86 × 10⁻¹¹ cm³·molecule⁻¹·s⁻¹. The calculated rate constants over the studied temperature range were used to fit the data and the three-parameter Arrhenius expression is obtained to be k(T) = 4.22 \times 10^ - 15 \cdot \left(\dfracT300\right)^1.44\cdot\exp \left(\dfrac2505T\right).