<正> Irradiation of the title compound with two D-labeled methyl groups gave 2-methyl-d3-4-(9-phenanthryl)-1-butene-d3,which was eluciated on the bases of the spectral evidences andchemical degradation.Based on the discussions of MO following and bond order change betweenthe designed basis orbital pairs on excitation,the most reasonable ring-opening mode of the titlecompound appeared to be the direct suprafacial 1,3-hydrogen migration.