A novel united-atom (UA) force field is proposed from our previously developed all-atom (AA) force field (J. Phys. Chem. B, 2004, 108, 12978.) for the imidazolium-based ionic liquids by the introduction of a coarse-grained method. The Lennard-Jones parameters for CH₂ and CH₃ in alkyls are fitted to match the AA force field, and the partial atomic charges are re-fitted by the one conformation two-step RESP method. The force field is verified by molecular dynamics simulations of pure ionic liquids and the mixture of [bmim][BF₄] and acetonitrile. The densities, self-diffusion coefficients, vaporization enthalpies, cohesive energy densities, and microscopic structures of both the pure components and mixtures are simulated. The simulated results from the UA force field agree well with those from the AA force field. In addition, the predictive capability of the UA force field for the liquid densities of [C_nmim][PF₆] is tested. The UA force field proposed in this work provides a useful tool with good accuracy and much less computational intensity for future molecular design of ionic liquids.