This work focuses on the use of waste cooking oil (WCO) as the main feedstock for hydrotreatment to produce biodiesel. In this study three parameters are considered for evaluating the effectiveness of this technology, pressure, hydrogen-to-oil (H-2/oil) ratio and liquid hourly space velocity (LHSV). For all experiments the same commercial hydrotreating catalyst was utilized. Hydrotreatment pressure, as a key parameter of hydrotreatment reactions, was initially studied via three experiments covering a range between 8.27 and 9.65 MPa. The H-2/oil ratio was examined via three experiments between 543 and 890 N m(3)/m(3) in order to evaluate its effect on biodiesel yield. Finally three different LHSVs (0.5, 1.0, and 1.5 h(-1)) were explored in order to study both hydrotreating catalyst effectiveness and catalyst life expectancy.