Without any additional organic solvent, the cyclotrimerization of aliphatic aldehyde could be catalyzed by acidic ionic liquid. Under optimum reaction conditions (298 K, 1 h, a molar ratio of 60:1 of isobutyraldehyde to ionic liquid), the conversion rate and selectivity for cyclotrimerization of isobutyraldehyde were 93.0 and 100%, respectively. The liquid product formed a separate phase that was decanted and the solid product could be separated by extraction. The ionic liquid could be easily reused after removal of water under vacuum.