Using waste-cooking-oil biodiesel as a renewable fuel for diesel engines can have several environmental benefits. However, the engine design parameters such as the compression ratio need to be optimized to achieve the best engine performance. The aim of this paper was to experimentally study the effect of changing the compression ratio on the engine performance and combustion characteristics of a compression–ignition engine fueled by different biodiesel/diesel fuel blends. It was found that the engine performance deteriorated when a high percentage of biodiesel was used in the fuel blend. However, the engine performance was significantly improved when the engine compression ratio was increased. The engine torque increased by 11.1 % when the engine was fueled by a biodiesel blend of 50 % at a compression ratio of 18 in comparison to the torque produced when the engine was fueled by the diesel fuel at a compression ratio of 14. Also, the engine brake thermal efficiency increased by 6 %. Both the ignition delay period and combustion duration decreased with the increase of engine compression ratio.