Abstract: The bromo-reaction of (2R, 4aS)-isolongifolenone to α-brominated isolongifolenone was investigated. Isolongifolenone was reacted with copper bromide in ethyl acetate, and the two isomerides, (2R,4aR,6R)-(+)-6-Br-isolongifolenone (αD25+81°) and (2R,4aR,6S)-(-)-6-Br-isolongifolenone (α D25-58°), were obtained by purification with preparative liquid chromatography. The structures of the two compounds were characterized by 1H NMR,FT-IR,GC-MS, specific rotation, and singal crystal X-ray diffraction. Furthermore, the effects of preparing conditions on the products were also explored. It was shown that (2R,4aR,6S)-(-)-6-Br-isolongifolenone could be obtained through a selective bromination of isolongifolenone by using ethanol as solvent and copper bromide as the reaction reagent. The optimum conditions of preparing (2R,4aR,6S)-(-)-6-Br-isolongifolenone were the molar ratio of copper bromide/isolongifolenone 3:1, ethanol dosage 60 mL, reaction time 3 h and reaction temperature 78℃. Under the foresaid conditions, the reaction selectivity and the yield of (2R,4aR,6S)-(-)-6-brominated isolongifolenone were 93% and 88%, respectively.