Abstract: Comparison of UV-VIS spectra between Fenton (UV-Fenton) and UV/H₂O₂ during the photo-degradation of phenol indicates that the Fenton process is not a simple hydroxyl radical reaction and multi-valent iron is the important intermediate during the Fenton process.Phenol was degraded through the inner-sphere electron transfer reaction within the complex Fe(Ⅳ)-phenol.The traditional hydroxyl radical theory seemingly could not be used to explain Fenton type oxidation reaction mechanism.