Provided are an ionic liquid and a method of preparing the same. The ionic liquid includes at least one type of compound represented by (Cat+)(R′COO−). Here, the Cat+ is a cation selected from the group consisting of quaternary ammonium, quaternary phosphonium, sulfonium, imidazolium, pyridinium, pyrazolium, piperidinium, pyrrolium, pyrrolidinium, triazolium, and a mixture of two or more thereof, and R′ is a hydrocarbon comprising at least one unsaturated bond, and having 4 to 30 carbon atoms. The ionic liquid is partially or completely miscible with various polar and/or non-polar solvents, and may be used as a solvent, a solvent additive, an electrolyte, a heat carrier, a charge carrier, a heat carrier additive, a charge career additive, or a phase transfer catalyst, at room temperatures and below. Furthermore, the cost of manufacturing the ionic liquid can be reduced.