An improved multi-dimensional model coupled with detailed chemical kinetics mechanism was applied to investigate the combustion and emission characteristics of a methanol/diesel reactivity controlled compression ignition (RCCI) engine. The fuel was supplied separately by directly injecting diesel fuel into cylinder well before top dead center, while premixing methanol through the intake port in the tested methanol/diesel RCCI engine. The effects of mass fraction of premixed methanol, start of injection (SOI) of diesel and initial in-cylinder temperature at intake valve closing (IVC) on engine combustion and emission were investigated in detail. The results show that both methanol mass fraction and SOI have a significant impact on cetane number (CN) distribution, i.e. fuel reactivity distribution, which determines the ignition delay and peak of heat release rate (HRR). Due to larger area with high-temperature region and more homogeneous fuel distribution with increased methanol, and the oxygen atom contained by methanol molecule, all the emissions are reduced with moderate methanol addition. Advanced SOI with high combustion temperature is favorable to hydrocarbon (HC) and soot reduction, yet not to the decrease of nitrogen oxide (NOx) and carbon monoxide (CO) emissions. Both increasing methanol fraction and advancing the SOI are beneficial to improve fuel economy and avoid engine knock. Moreover, it was revealed that the initial temperature must be increased with increased methanol fraction to keep the 50% burn point (CA50) constant, which results in decrease of the equivalent indicated specific fuel consumption (EISFC) and all emissions, except for slight increase in NOx due to the higher burning temperature. (C) 2013 Elsevier Ltd. All rights reserved.