To better understand the efficient transformation of glucose to fructose catalyzed by chromium chlorides in imidazolium-based ionic liquids (ILs), density functional theory calculations have been carried out on a model system which describes the catalytic reaction by CrCl2 in 1,3-dimethylimidazolium chlorine (MMImCl) ionic liquid (IL). The reaction is shown to involve three fundamental processes: ring opening, 1,2-H migration, and ring closure. The reaction is calculated to exergonic by 3.8 kcal/mol with an overall barrier of 37.1 kcal/mol. Throughout all elementary steps, both CrCl2 and MMImCl are found to play substantial roles. The Cr center, as a Lewis acid, coordinates to two hydroxyl group oxygen atoms of glucose to bidentally rivet the substrate, and the imidazolium cation plays a dual role of proton shuttle and H-bond donor due to its intrinsic acidic property, while the Cl- anion is identified as a Bronsted/Lewis base and also a H-bond acceptor. Our present calculations emphasize that in the rate-determining step the 1,2-H migration concertedly occurs with the deprotonation of O2-H hydroxyl group, which is in nature different from the stepwise mechanism proposed in the early literature. The present results provide a molecule-level understanding for the isomerization mechanism of glucose to fructose catalyzed by chromium chlorides in imidazolium chlorine ILs.