The study investigated the characteristics of the combustion, the emissions and the thermal efficiency of a direct injection diesel engine fuelled with neat n-butanol. Engine tests were conducted on a single cylinder four-stroke direct injection diesel engine. The engine ran at 6.5 bar IMEP and 1500 rpm engine speed. The intake pressure was boosted to 1.0 bar (gauge), and the injection pressure was controlled at 60 or 90 MPa. The injection timing and the exhaust gas recirculation (EGR) rate were adjusted to investigate the engine performance. The effect of the engine load on the engine performance was also investigated.The test results showed that the n-butanol fuel had significantly longer ignition delay than that of diesel fuel. n-Butanol generally led to a rapid heat release pattern in a short period, which resulted in an excessively high pressure rise rate. The pressure rise rate could be moderated by retarding the injection timing and lowering the injection pressure. The applicable window of the injection timing for the n-butanol fuel was much narrower than that of the conventional diesel fuel because of the constraints of misfiring and excessive pressure rise rate. At 6.5 bar IMEP, the n-butanol combustion produced near zero soot and very low NOx emissions even at a low EGR rate. However, the NOx emissions increased at higher engine loads. Nevertheless, the NOx emissions could be reduced to 0.2 g/kWh (indicated) by increasing the amount of the EGR rate while maintaining near zero soot emission. The study demonstrated that the n-butanol fuel had the potential to achieve ultra-low exhaust emissions. However, challenges of improving the ignition controllability and lowering the pressure rise rate were identified.