Experimental study was conducted on a compression ignition engine to investigate into the effects of methanol substitution percent (MSP), injection timing, intake air temperature (TO, exhaust gas recirculation (EGR) and other parameters on the ultrafine particle emitted from the engine. The testing engine was run on the mode of Diesel/methanol dual fuel, which was modified from a turbocharged, inter-cooling diesel engine. The testing results showed that tailpipe length had a significant effect on the nucleation mode particles but limited effect on accumulation mode particles. The effect of MSP on ultrafine particle was distinctly dependent on injection timing. Under retarded injection timing condition, total particle number gradually reduced with the increment of MSP. But it changed little or even increased under early injection timing condition. In addition, the injection timing and T-i had combined effects on ultrafine particle. Under high T-i condition, accumulation mode particles number significantly increased with delaying injection timing, while it increased slightly under low T-i condition. Nucleation mode of particles exhibited opposite trend. And at any rate of given EGR addition, soot and PN remained low in DMDF mode with high MSP and the soot/PN-bump phenomenon will be weakened as methanol increases. (C) 2016 Elsevier Ltd. All rights reserved.