In this study, bioethanol produced from sugar beet and biodiesel produced from waste cooking oils was blended with each other in the volumetric rates to be 20% and 80%, respectively. Cetane improver (di-tert-butyl peroxide) was added into blend fuel in 1–2–3% as volumetric. The obtained blends were used as a fuel in the diesel engine which is single-cylinder, direct-inject, four-stroke. The tests were conducted under four different engine loads (25%, 50%, 75% and 100%) and 1400 rpm engine speed. The test results showed that brake-specific fuel consumption was decreased up to 15.5% thanks to the addition of cetane improver, although biodiesel and bioethanol increased brake-specific fuel consumption by 16.1% and 27.54%, respectively. However, brake thermal efficiency values were increased up to 9.44% with both biodiesel and cetane improver added blend fuels, while brake thermal efficiency was decreased by 3.88% with bioethanol addition. The more compatible combustion characteristics with that of diesel fuel have been obtained due to especially the increase in cetane number. The use of biofuels increased both maximum cylinder pressure and heat release rate. While, with the cetane improver addition, a decrease in CO, HC and smoke opacity values was observed up to 22.5%, 17.44% and 24.44%, respectively, CO2, NOX and exhaust gas temperature values were increased up to 19.55%, 5% and 15.22%, respectively, according to bioethanol blend fuel.