Octatetrayne monomers with methane groups, i.e. R-C=C-C=C-C=C-C=C-R in which R=-(CH2)(m)-OCONH-CH2-COO-(CH2)(n)-H (5ECMU-4A : m=5, n=2; 3BCMU-4A : m=3, n=4), were synthesized, and their polymerization behavior and optical properties were investigated, The polymer structures were confirmed to be diacetylene-substituted polydiacetylenes like polymers from octatetraynes studied previously. Though both of thin film polymers obtained by gamma-ray and UV irradiation showed excitonic absorption around 680 nm, absorbance of gamma-ray irradiated one was three times larger than that of W-irradiated one. Thus, it was found that UV-irradiation caused not only polymerization but also deterioration of the produced polymers, and gamma-ray irradiation is superior to UV irradiation for polymerization. The third-order nonlinear optical susceptibilities (chi((3))(-3 omega) s) of these polymers were (0.56-3.7)x10(-11) esu, and roughly proportional relation between chi((3)) s in three-photon resonant region and absorbance were found in this series of polymers.