Quasi-diabatic band theory has been developed by using the localized crystal orbital (LCO) transformation to study the electronic structure representing the diabatic behavior of electrons. The theory has been applied to nondoped alternating trans-polyacetylene (t-PA) to understand better the electronic structure from this new standpoint. The results show that the pi- (pi*-) band of the newly constructed crystal orbital becomes higher (lower) than that of the canonical crystal orbital. The occupation number analysis also indicates that the pi-bands near the Fermi energy level play a significant role in the electron transport. These results have been discussed in terms of the LCOs representing the biradical and zwitterionic states.