As a biomass-based renewable fuel, n-butanol has many superior properties, such as better miscibility in diesel, higher energy content and higher cetane number, which make it an attractive alternative or blending component to diesel fuel compared with its alcohol competitors, methanol and ethanol. Although n-butanol has so many advantages, literature on the effects of its addition on diesel engines are still not sufficient. In this study, the influences of n-butanol addition on the performance and emissions of a turbocharged common-rail diesel engine are investigated under different speed and load conditions. The fuel consumption rate and the gaseous emissions (CO, CO₂, HC, and NOx) are measured. Moreover, an electrical low pressure impactor (ELPI) is utilized to study the particulate matter (PM) number-size distributions under different conditions. The results show that as more n-butanol is added, the brake fuel specific consumption is increased while only a slight variation of the fuel conversion efficiency is found. As for the gaseous emissions, the concentration of CO and CO₂ remain almost unchanged, the HC emission increases obviously, and the NOx emission is slightly reduced except for the high-speed-high-load condition. Under a specific speed-load condition, the peak number concentration of PM decreases apparently with the increase of the n-butanol blending ratio. Moreover, the peak number size of PM becomes smaller when n-butanol is added under the high-load conditions. The results indicate that n-butanol has the merits to reduce the PM emission without significantly affecting the performance and the other emission characteristics of a diesel engine. However, further investigations are still required to evaluate and address the potential problems that may be caused by the use of n-butanol, such as the emissions of toxic oxygenated compounds, the unacceptable in-tank flammability limit and the lower fuel lubricity.