A valence bond method, namely the bonded tableau unitary group approach, is applied to analyze the pi electron delocalization of benzene, cyclobutadiene, and butadiene, and the resonance energies are also calculated and extensively compared with experimental data and theoretical results in the literature. In the frame of ab initio calculations, we optimize the geometries of hypothetical molecules with localized nonresonating double bonds without any artificial approximation. Our results show that the Csp(2)-Csp(1) single bend length (1.509 Angstrom with the 6-31G basis set) is only about 0.021 Angstrom shorter than the Csp(3)-Csp(3) bond, and the delocalization is a driving force in conjugated systems. If the delocalization energy can compensate the energy needed by the shortening of some Csp(2)-Csp(2) bonds, the system will prefer a regular geometry with uniform C-C bond lengths, otherwise the system will be stable toward an alternate geometry where delocalization is still important. An interesting result is that even in cyclobutadiene the delocalization is energetically beneficial and the theoretical resonance energy is 3.16 kcal/mol with the STO-6G basis set or 5.67 kcal/mol with the 6-31G basis set, which is close to the value of butadiene. Moreover, the pi orders of the long bonds in C4H4 and C4H6 are very close. Thus it can be concluded that the antiaromacity of C4H4 is a direct outcome of the sigma frame’s ring strain rather than pi electronic delocalization.