Acylation of acenaphthene with benzoyl chloride to 5-benzoylacenaphthene catalyzed by [Bmin]Cl/AlCl3 ([Bmin]+ = 1-butyl-3-methylimidazolium cation) ionic liquid was investigated. Pure 5-benzoylacenaphthene was obtained and the structure of 5-benzoylacenaphthene was identified by GC/MS, FT-IR and 1H NMR spectra. The yield of 5-benzoylacenaphthene was up to 85.2 % and the selectivity towards 5-benzoylacenaphthene was up to 89.6 %. The [Bmim]C1/A1C13 ionic liquid catalyst shows well catalytic activity after running for 5 times. The facile product separation and the recycling performance of the ionic liquid catalyst is expected to contribute to the development of clean and environmentally friendly strategy for the synthesis of 5-benzoylacenaphthene.