Serum paraoxonase 1 (PON1) is a versatile enzyme for the hydrolysis of various substrates (e.g., lactones, phosphotriesters) and for the formation of a promising chemical platform gamma-valerolactone. Elucidation of the PQNL-catalyzed,lactonase reaction mechanism is very important for, understanding the enzyme function and for engineering this enzyme for specific applications. Kinetic study and hybrid quantum mechanics/molecular mechanics (QM/MM) method were used to investigate the PONT-catalyzed lactonase reaction of gamma-butyrolactone (GBL) and (R)-gamma-valerolactone (GVL). The activation energies obtained from the QM/MM calculations were in good agreement with the experiments. Interestingly, the QM/MM energy barriers at MP2/3-21G(d,p) level for the lactonase of GVL and GBL were respectively 14.3-16.2 and 11.5-131 kcal/mol, consistent with the experimental values (15.57 and 14.73 kcal/mol derived from respective k(cat) values of 36.62 and 147.21 s(-1)). The QM/MM energy barriers at MP2/6-31G(4) and MP2/6-31G(d,p), levels were also in relatively good agreements with the experiments. Importantly, the difference in the QM/MM energy barriers at MP2 level with all investigated basis sets for the lactonase of GVL and GBL were in excellent agreement With the experiments (09-3.1 and 0.8 kcal/mol, respectively). A detailed mechanism for the PON1-catalyzed lactonase reaction was also proposed in this study.