Methanol and biodiesel, which have high oxygen content, are high-quality alternative fuels of diesel engine. If blended, the combustion process of diesel engine would be improved and the particulate matter would be reduced effectively. However, with the increasing of methanol blending ratio, the cetane number of methanol/biodiesel blend would decrease a lot, causing the problem of poorer self-ignitability. In this study, three kinds of cetane number improvers with different chemical components are chosen. The cetane number improvers are 2-ethylhexyl nitrate, cyclohexyl nitrate and 2-methoxyethyl ether, which are added to the methanol/biodiesel blend with the ratio of 0.3%, 0.3% and 3% respectively. The cetane number of the blend increases from 45.5 to 63.5. In this paper, the physical - chemical properties of the blend are reported, and the effect of the improvers on engine performance in a direct-injection diesel engine is studied. The result shows that the improvers, especially cyclohexyl nitrate, have positive function on shortening the ignition delay period and extending the combustion duration. Besides, the maximum in-cylinder pressure of the blend with improvers increases from 6.58% to 9.94% and the maximum heat release rate of the blend with improvers increases from 1.38% to 6.78%. At rated condition, the three improvers reduce NOx concentration from 3.87% to 12.90% and smoke from 11.76% to 38.24%, but at the same time, they cause another problem-the increasing of HC and CO concentration. Among these improvers, 2-methoxyethyl ether has the best effects on the reduction of NOx concentration and smoke of methanol/biodiesel. In addition, viscosity and density of the blend are rarely impacted by the improvers. The brake thermal efficiency and exhaust temperature maintain the same level of methanol/biodiesel. (C) 2014 Elsevier Ltd. All rights reserved.