The Friedel-Crafts acylation reaction between 5-cyanoindole and 4-chlorobutyryl chloride was conducted in ionic liquid 1-buty-3-methylimidazolium chloroaluminate ([bmiCl/AlCl3), the major product was 5-cyano-3-(4-chlorobuty) indole. The structure of the product was characterized by Mass and1H NMR spectra. When the molar ratio of AlCl3to [bmiCl in [bmiCl/AlCl3is at 1:1 or less, no 5-cyano-3-(4-chlorobuty) indole could be obtained. The suitable molar ratios of AlCl3to [bmiCl in [bmiCl/AlCl3were from 2:1 to 3:1, the yields of the major product were from 68% to 70%, respectively. When the molar ratio of [bmiCl/AlCl3to 5-cyanoindole was less than 2:1, the acylation reaction was difficult to carry out, because 5-cyanoindole would be not totally dissolve in [bmiCl/AlCl3under the situation.