A series of new N-butylammonium carboxylate ionic liquids (N-butylammonium formate, N-butylammonium acetate, N-butylammonium propionate, and N-butylammonium butyrate) were synthesized by a one-step method and characterized. The thermodynamic properties such as surface tension, density, electrical conductivity, and dynamic viscosity of the ILs were measured as functions of temperature. Important parameters such as molecular volume, crystal energy, thermal expansion coefficient, standard molar entropy, and the surface properties are estimated by the empirical equations. The Vogel-Fulcher-Tamman (VFT) equation is used to explore the dependence of electrical conductivities and dynamic viscosities on temperature. The relationship between the molar conductivity and the fluidity of the ILs was examined through the use of Walden plots. To describe intermolecular interactions of the N-butylammonium carboxylate ILs, the optimized structures and energetics of ILs are calculated by DFT calculations. The calculated energies showed that the increase in the interaction energies between the ion clusters is the same as the trend of experimental viscosities.