Investigation of conformational isomerism of ring compounds can help us get a clear comprehension of the ring structure and reveal significant structure-activity relationship. In this study, conformational isomerism of the cationic moiety of ionic liquid 1-ethyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C(2)C(1)C(1)(4)pi][NTf2]) has been investigated by means of H-1 nuclear magnetic resonance spectra. The energy levels for different conformations of the cationic moiety [C(2)C(1)C(1)(4)pi](+) are obtained via density functional theory calculations. The predominant cis-conformer in [C(2)C(1)C(1)(4)pi][NTf2] at its liquid state is observed under ambient conditions, where the ethyl group locates at the equatorial position of quaternary nitrogen atom, consistent with the calculated results. The trans-conformer minorities in the IL convert to the cis-conformers when [C(2)C(1)C(1)(4)pi][NTf2] is well crystallized. Besides, the addition of polar solvents, such as ethanol, leads to a convenient and complete transformation from the trans-form to the recognizable cis-form. The phase-transition behaviors have been measured by means of differential scanning microcalorimetry (DSC), and the DSC results can be highly affected by the initial state of the IL. Density and viscosity measurements for mixtures of [C(2)C(1)C(1)(4)pi][NTf2] with ethanol or 1-propanol at different temperatures T = (293.15 to 323.15) K are performed. Conformational isomerism affects the excess molar volumes of [C(2)C(1)C(1)(4)pi][NTf2] + alcohol systems more significantly than the viscometric property. The behaviors, as comparison, for the mixtures of 1-n-pentyl-1,4-dimethyl-piperazinium bis(trifluoromethylsulfonyl)imide ([C(5)C(1)C(1)(4)pi][NTf2]) with ethanol are observed with the same phenomena as the common binary systems. On the basis of the experimental and calculated results of the ILs, it can be concluded that conformational isomerism in the cation of [C(2)C(1)C(1)(4)pi][NTf2] is quite significant, and it should be taken into account when sensitive properties are evaluated.