In the course of our study on the ene reaction of trifluoromethyl ketones, a trifluoromethyl group has been observed to behave as a larger substituent than commonly believed in the biomedicinal field. To estimate the steric effect of a trifluoromethyl group, we synthesized several trifluoromethyl ketones (RCOCF)₃ and examined in detail their ene reaction with cyclohexene, a 1, 2-disubstituted ene having the least steric requirement. In this reaction, a trifluoromethyl group was found to behave as if it were a much larger substituent than a phynyl or isobutyl group and as large as a sec-butyl group.