The general expressions we previously derived for calculating of the vibronic structure in electronic absorption spectrum of a large polyatomic molecule based on frequency-domain approach (J. Molec. Spectrosc.2013, 286-287, 30-45) are applied to simulate the vibronic structure in the dipole-allowed g u 11A ®11B electronic transition of trans-1,3,5-hexatriene molecule. The calculated results indicated that spectral profiles are primarily described by the Franck-Condon progression of four totally symmetric modes, 5 n , 10 n , 11 n and 13 n , among which the 5 n (CC double bond stretch) and 10 n (CC single bond stretch) are the longest progression-forming modes, while the modes 11 n and 13 n have much less activities. The spectra also show combination bands (as 51111 , 51101 , etc.) produced by the g a modes.