Several authors have indicated that PCP is a toxic chemical and recalcitrant to biodegradation. AOPs is one of the most effective process for degradation of persistent compound. Since the mineralization of recalcitrant compound by AOPs (Advanced Oxidation Processes) often requires long reaction time and strong doses of oxidant, the combination of this process with biological one, considered as an efficient and economic method. In this work degradation of pentachlorophenol in aqueous solution with fenton reagent (H₂O +Ferrous ion) was studied. The experiment was done in batch mode, and the initial concentration of PCP was 0.055mM, in pH=3, H₂O₂=0.6mM, Fe=0.2mM, more than 95% of PCP was degraded in first minute after the reaction was started. Therefore this reaction is very fast and in the initial phase degradation of PCP follows first order kinetics and kineticts constant (K) was 0.026 (S^-1). Chloride ion generation as PCP degradation by product was investigated and it was found that the scavenging effect of chloride is negligible. PH and UV₂₁₅ absorbance analysis, after reaction completion, indicated that generated intermediates have the less chlorinated nature, acidic properties and nonphenolic structure. Chloride ion increases from 0 mg/L to 6 mg/L, pH decreases from 3 to 2.82 and UV₂₁₅ absorbance decreases from 0.48 to 0.1, therefore it can be resulted that their biodegradability modified and their recalcitrance reduced. In the long time reaction (10hr) experiments, TOC and COD analysis indicated that PCP did not mineralize and TOC and COD reduction was only 20% and 30% respectively. Results from this study indicated that scavenging effects of generated intermediate is important in high doses of H₂O₂.