We have calculated TPA cross section delta value of dithienothiophene (DDT)-based molecules to gain insight into the origin of the large delta values. The energy and transition dipole moments for the ground state and multiple excited states were calculated by combining the Hartree-Fock Hamiltonian with single configuration interaction formalism. For the calculation of the imaginary part of the third-order polarizability the sum-over-state expression was used. There are some differences between theoretical and experimental values. However the theoretical calculation reasonably estimates the delta values of the DTT-based molecules roughly 2-10 times larger than those fluorene- as well as stilbene-based ones.