Methyldopa was selected to be degraded by Fenton's reagent in the experiment. The experimental results showed that it was feasible to the removal of Methyldopa and COD by Fenton's reagent. The mechanism of Methyldopa removal by Fenton's reagent was significantly different according to the Fe2+ :H2O2 ratio. With high ratio of Fe2+ :H2O2 (> or = 2), the Fenton reaction was coagulation enhanced by H2O2. With medium ratio of Fe2+ :H2O2 (= 1), the Fenton reaction could be characterized into two specific systems : oxidation and coagulation. With low ratio of Fe2+ :H2O2 (< or = 0.2), the Fenton reaction was oxidation, including H2O2 oxidation catalyzed by Fe2+ and degradation by a hydrated ferryl-complex Fe(aq)4+. With the analysis of the mechanism according to low ratio of Fe2+ :H2O2 (< or = 0.2), a kinetics model was adopted to describe the reaction, and the rate equation could provide the evidence for the main reaction pathway, which was fitted very well with the experiment data for the relative error below 10%. It was showed that the kinetic models could primary describe the process of the removal of Methyldopa and COD by Fenton's reagent.