DFT calculations have been performed to gain insight into the mechanism of production 5-hydroxymethylfurfural (HMF) from fructose catalyzed by a representative imidazolium-based ionic liquid (IL), 1-butyl-3-methyl-imidazolium bromide ([BMIM]Br). The overall barrier of the catalytic conversion is found to be 41.5 kcal/mol, which is in contrast to 75.6 kcal/mol located in previous literature. Different from the mechanism proposed in the literature, which Br anion of IL interacts with the hydroxyl group of the substrate via H-bonding to induce the dehydration process, while cation of IL acts as a spectator, the present calculations emphasize the collaboration of cation and anion of IL.