Photo-induced isomerization of 1, 2-di (3-aminophenyl)-1, 2-diphenylethene contains following reactions; 1) rotation of m -aminophenyl groups, 2) epimerization, 3) rotation of C=C double bond part. The details of these reaction processes were investigated by means of DFT calculation (B3LYP) with LANL2DZ basis set. The activation energies for the rotation of C-C single bond of m -aminophenyl group and for the epimarization of the molecule were 8.8 kcal/mol and 7.9 kcal/mol, respectively. Although large activation energy requires for the rotation of the C=C double bond at singlet states, C=C double bond lengthen and close to rotatable C-C single bond with radical on each carbons at trilet states.