We synthesized a series of FeCl3/[C(4)mim]Cl (iron(III) chloride with 1-butyl-3-methylimidazolium chloride) ionic liquids. The temperature dependence of the Raman spectra of the FeCl3/[C(4)mim]Cl ionic liquids was measured for the first time to analyze their ionic species. In addition, the infrared spectra, thermodynamic properties, and freeze-fracture transmission electron microscopy were combined together with the Raman spectra to reveal the microscopic information of the FeCl3/[C(4)mim]Cl ionic liquids. When the mole ratio of FeCl3/ [C(4)mim]Cl is less than 1, the Raman scattering identifies the presence of [FeCl4] in the ionic liquid. When FeCl3 is in excess, [Fe2Cl7](-) begins to appear. Besides, we found that the relative intensity of the two symmetry vibrations of [Fe2Cl7](-) is changing as the temperature is varied. The strength of the interionic interactions in FeCl3/[C(4)mim]Cl ionic liquids follows the order 1/1.5 > 1.5/1 > 1/1 due to the formation of [FeCl4](-) and [Fe2Cl7](-). The nanostructures in FeCl3/[C(4)mim]Cl ionic liquids were observed for the first time by using biological imaging. The sizes of the local domains are found to be tens of nanometers.