Starting from the D2h structures of disilene and ethylene, the potential energy surfaces of the ground and excited singlet ( π → π) states along the rotational coordinate (au) and the trans-bending coordinate (b28) and the transition moments between these two states are calculated by using the multiconfigurational self-consistent-field (MCSCF) method with the 3-21G(d) basis set. Based on these results, the vibrational wave functions of these two states are expanded in Fourier series, and the Franck-Condon factors and the oscillator strengths are calculated between the vibrational levels of these two states. Though the transition moments are not constants along these vibrational coordinates, both Franck-Condon factors and the oscillator strengths exhibit the same trends. Therefore, the Franck-Condon principle should be approximately valid.