The purpose of this work is to evaluate the degradation rate of contaminants in modified Fenton's reagent using chelating agents (NTA, EDTA, citrate and oxalate) and to investigate coexisting oxidative-reductive mechanism under various pH and H2O2 concentrations. The modified Fenton's reaction using iron(III)-NTA showed maximum degradation of probe compounds: 1-hexanol and carbon tetrachloride (CT). In the modified Fenton's reaction, degradation of probes was higher at high concentration of H2O2. As the pH was increased from 3 to 9, the degradation rate of probes increased. However, the degradation of probes decreased significantly at pH 12. Normal Fenton's reaction was also conducted in parallel with the modified Fenton's reaction. The degradation of 1-hexanol was effectively processed at pH of 2 ~ 4. As pH was increased, the degradation of CT also increased. To confirm that the modified Fenton's reagent provided not only hydroxyl radicals but also reductants, the oxidation and reduction mechanisms were further studied by scavenging analyses using isopropanol as the hydroxyl radical scavenger and chloroform as the reductant scavenger. These results indicated that hydroxyl radical was the primary oxidant for 1-hexanol decomposition, whereas the degradation of CT was linked to the reductants, superoxide radicals (OO(-)·) and hydroperoxide anions (OOH(-)).