In this work, the commercially available diesel fuel (D), diesel (80%)-microalgae biodiesel (20%) (by volume) (D80B20), diesel (70%)- microalgae biodiesel (20%)- butanol (10%) (D70B20But10) and diesel (60%)- microalgae biodiesel (20%)- butanol (20%) (D60B20But20) fuels were tested to evaluate the effects of the fuel blends on the performance and exhaust emissions of a diesel engine. Engine performance parameters and exhaust gas emissions such as nitrogen oxides, carbon monoxide and smoke opacity were measured. The results showed that; although butanol addition caused a slight reduction in torque and brake power values, the emission values of the engine were improved. Therefore, butanol can be used as a very promising additive to diesel- microalgae biodiesel blends. (C) 2014 Elsevier Ltd. All rights reserved.