This work deals with the separation of 1-hexene and n-hexane as the representation of olefins and paraffins with ionic liquids, as well as N-methyl-2-pyrrolidone (NMP) screened by computer-aided molecular design (CAMD). On the basis of conformation analysis of solvents and ionic liquids, the conductor-like screening model for real solvents (COSMO-RS) was used to make a priori prediction for suitable ionic liquids. It was found that the suitable ionic liquids should have small molecular volume, unbranched group and sterical shielding effect around anion charge center. Headspace-gas chromatography (HSGC) experiments were done at 313.15 and 333.15 K. It was verified that the anion with sterical shielding effect around anion charge center is favorable for increasing the selectivity, and [C(8)Chin](+)[BTA](-) is the best among all the ionic liquids investigated. The separation mechanism of olefins and paraffins with ionic liquids can be explained by the theory of Prausnitz and Anderson's solution thermodynamics. This work also can be extended to the separation of other hydrocarbons with ionic liquids since the separation mechanism between n-hexane/1-hexene and other hydrocarbons is consistent. (c) 2006 Elsevier B.V. All rights reserved.