An all-atom force field for a class of the room temperature ionic liquids of the 1-alkyl-3-methylimidazolium cation family was developed. The model is based on the AMBER force field with modifications on several parameters. The refinements include three aspects. (1) The force coefficients of the bond and angle parameters were adjusted to fit the vibrational frequency data, from both experiment and ab initio calculations. (2) The parameters for two types of torsions, which are absent in the original AMBER, were obtained by fitting the torsion energy profiles depending on dihedral angles. (3) The results of the minimum interaction energies and geometries for several ion pairs, calculated from ab initio and the force field, respectively, are compared. Then, the van der Waals (VDW) diameter of a type of hydrogen atom (H5) is adjusted. To validate the force field, we performed molecular dynamics (MD) simulations for five RTILs. The predicted densities are in better agreement than those reported from other simulations. The space distribution functions (SDFs) obtained from MD are visualized to depict the microscopic structures of these liquids. The internal energy components and the self-diffusion constants are also discussed.