Because of environmental concern, wastewater containing nitrophenols must be treated before it is allowed to discharge. In addition to traditional treatment methods, advanced oxidation processes (AOPs) using a powerful hydroxyl radical to decompose organic pollutants in wastewater receive great attraction. Among the many AOPs to degrade nitrophenols, the Fenton reaction was proven to be the most effective and the least expensive method, and the resultant solutions treated by the Fenton reagent were not toxic. However, owing to the complicated mechanism involved in the Fenton reaction, an oversimplified first-order rate equation or empirical rate laws were usually used to model the Fenton reaction. Such oversimplified or empirical kinetics cannot be used confidently in the design and operation of large-scale Fenton processes. A series of experiments were carried out to determine the initial decomposition rates of p-nitrophenol (PNP) at varying initial ferrous ion, hydrogen peroxide, and PNP concentrations. The PNP decomposition rate equations for two proposed mechanisms were derived by the general rate methodology. The rate equation resulting from the reaction mechanism involving the PNP reaction with both hydroxyl and hydroperoxy radicals fits the experimental data well.