Abstract: Compared to gasoline engine, the thermal efficiency of diesel engine is higher and the 30% fuel efficiency can be achieved, but the particle emission is very serious. Particles which contain soluble organic fraction (SOF), soot and inorganic salt are mainly the result of incomplete combustion or pyrolysis of fossil fuels and other organic materials. According to the particle size distribution, the exhaust particles include ordinary particles (with size more than 100 nm) and ultrafine particles (with size less than 100 nm). And ultrafine particles include nuclei mode particles (with size less than 50 nm) and accumulation mode particles (with size between 50 and 100 nm). Numerous studies have shown that the particles are harmful to human and environment, and they may cause cancer and other diseases. Formerly, the emission regulations only restricted the PM (particulate matter) mass concentration, but recently a series of policies have included the limitation of particulate number concentration. The particle emission of diesel engine has gradually been paid extensive attention by many researchers. Burning diesel mixed with methanol could improve combustion and reduce the emissions of particles, but the physicochemical properties of diesel and methanol are greatly different, and they are mutually immiscible. Generally, forming micro-emulsion fuels by mixing them with cosolvent is the most efficient way. At present, a series of achievements have been gained at home and abroad, but the researches about using the mixture of methanol and biodiesel are still few, especially about the size distribution of fine particles. As a kind of green renewable energy, biodiesel can improve the emission characteristics of engine; also, it could contribute to the formation of methanol-blending biodiesel micro-emulsion fuel. With the use of combustion analyzer and Engine Exhaust Particle Sizer 3090 spectrometer, the combustion process and the particulate number concentration distribution characteristics were studied with a diesel engine fuelled with methanol-blending biodiesel micro-emulsion fuel. Results showed that compared to the engine fuelled with blending biodiesel B20 (20% biodiesel and 80% diesel in volume fraction), the in-cylinder pressure, pressure rise rate and heat release rate curves for the engine fuelled with methanol-blending biodiesel micro-emulsion fuel were obviously retarded, and the maximum pressure rise rate and heat release rate increased; the particle presented nuclei mode and the size was mostly between 6 and 22 nm at low load, and the total number of nuclei mode particles showed that the B20 fuel was the most, followed by B20M5 (95% B20 and 5% methanol in volume fraction), B20M10 (90% B20 and 10% methanol in volume fraction) and B20M15 (85% B20 and 15% methanol in volume fraction) in order; at high load, the particulate number concentration distribution was unimodal with the size between 6 and 275 nm, the particle morphology was accumulation mode, and the peak of particulate size distribution was around 100 nm. The total particle numbers of nuclei mode particles, accumulation mode particles and all particles for B20 were the most. The total number of accumulation mode particles for B20 was 2.45 times that of B20M5, 2.05 times that of B20M10, and 3.47 times that of B20M15. And the total number of all particles was 2.26, 1.98 and 3.77 times that of B20M5, B20M10 and B20M15 respectively. With the increase of methanol fraction, the percentage of nuclei mode particles rose and the percentage of accumulation mode particles decreased, and the sum of exhaust particle declined. Relative to blending biodiesel, the more oxygen the micro-emulsion fuels contained, the more proportion the ultrafine particles occupied. So, the mixing proportion of methanol should not be too high. The research provides a reference for the combustion and the particle emission control of methanol-biodiesel blending fuel.