Palm oil (PO) was treated using different methods in order to use and test it as fuel in Compression Ignition (Cl) engines. The treatments include PO preheated and preparation of PO/diesel oil blends, using mixtures of PO with waste cooking oil (WCO), which are converted into esters by a transesterification process. The purpose of this study is to evaluate the potential of the palm oil-based biofuels to replace diesel oil in Cl engines. Tests were conducted in a single cylinder, four-stroke, air-cooled, direct injection diesel engine (no engine modifications were required). Experiments were initially carried out with diesel oil for providing baseline data. All the tested fuels have a low heating value compared to diesel fuel. A high fraction of PO in diesel fuel decreases the heating value of the blend. The brake thermal efficiency increases for the PO/Diesel blends. HC emissions for all those fuels except for the PO/Diesel blends are found lower, while CO emissions rise for all types of fuels. NOx emissions are higher at low load, but lower at full load, for the engine fueled with PO and lower both at middle and full load for the engine fueled with the esters. (C) 2011 Elsevier Ltd. All rights reserved.