A thorough thermodynamic analysis of different DBP conversion paths induced by hydroxyl (center dot OH) and sulfate radicals (SO4 center dot-) is performed using Density Functional Theory (DFT) calculations in the gas and aqueous phases. We propose the thermodynamics favorable reaction chains for the major degradation products formation, and the involved reaction mechanisms include radical adduct formation (RAF), formal hydrogen atom transfer (FHAT), center dot OH oxidative cleavage of C = C double bond, and acid-catalyzed decomposition of C-C single bond. Theoretical results indicate that phthalic anhydride (PA) and acetophenone (ACP) are respectively the dominant and minor products in water, which is consistent with the experimental data. Computational results reveal that the reactivity of center dot OH for the H-abstraction reaction is higher than that of SO4 center dot-. Interestingly, the unsaturated C = C bonds cleavage reaction can directly occur with center dot OH attack in aqueous solutions. However, it needs to overcome an extra radical adduct formation step when it is attacked by SO4 center dot-. The present work opens a new window to illustrate the reaction mechanism and reactivity of % OH and SO4 center dot- toward different chemical structures of organics from the view of molecular level.