A new pi-conjugated polymer, poly(2,5-diheptyl-1,4-phenylenehexa-1,3,5-trienylene) (HpP-PHT) was synthesized by a Wittig reaction. The polymer was obtained as a red solid (lambda(max) = 453 nm in chloroform) that was easily soluble in common organic solvents, as a result of introducing long alkyl side chains onto the aromatic rings. The structure was confirmed by IR and NMR spectra. The degree of polymerization was shown by GPC and NMR peak integrals to be about 10. The third-order nonlinear susceptibility chi((3)) for a thin film of HpPPHT was evaluated to be 2.3 x 10(-12) esu (1.98 mu m, nonresonant region) by third-harmonic generation (THG). The value was about 3 times higher than that of poly(2,5-diheptyl-1,4-phenylenevinylene) (HpPPV) due to the more effective pi-delocalization in HpPPHT.