Abstract: From superacid POSA (perfluorooctanesulfonic acid) catalyzed esterification of p-substituted benzoic acids with ethanol, we got not only the rate canstants k, the energy of activation ΔE, the entropy of activation ΔS~≠, and the enthalpy of acti-vation ΔH~≠; but also a good Hammett linear Free energy relationship (LFE Rela-tionship). This is the first report of applying LFE relationship to superacid catalysis. Finally, the reaction intermediate, the reaction constant ρ, and the mechanism of the reaction were also discussed. It is deduced by this paper that the mechanism of POSA catalyzed esterification is basically identical with those of normal strong acids catalysis, and the reaction is a process of general acid catalysis.