Olive mills wastewater (OMW) is characterized by its high organic content and refractory compounds. In this study, an advanced technology for the treatment of the recalcitrant contaminants of OMW has been investigated. The technique used was either photo-Fenton as homogeneous photocatalytic oxidation or UV/semi-conductor catalyst (such as TiO(2), ZrO(2) and FAZA) as heterogeneous photocatalytic oxidation for treatment of OMW. For both the processes, the effect of irradiation time, amounts of photocatalysts and semi-conductors, and initial concentration of hydrogen peroxide has been studied. At the optimum conditions, photo-Fenton process achieved COD, TOC, lignin (total phenolic compounds) and total suspended solids (TSSs) removal values of 87%, 84%, 97.44% and 98.31%, respectively. The corresponding values for UV/TiO(2) were 68.8%, 67.3%, 40.19% and 48.9%, respectively, after 80 min irradiation time. The biodegradability expressed by BOD(5)/COD ratio for treated wastewater was ranged from 0.66 to 0.8 compared to 0.19 for raw wastewater indicating enhancement of biodegradation.