To understand the influence on the thermal conductivity by the length of the molecular chain in the polymer fiber, the thermal conductivity and thermal diffusivity of ramie fibers and those irradiated by gamma rays, which induced molecular chain scission of cellulose, were investigated in a range of low temperatures. The degrees of polymerization, crystallinities, and orientation angles of ramie fibers and those irradiated by gamma rays (gamma-ray treatment) were measured by the solution viscosity method, solid-state NMR, and X-ray diffraction. Only the degree of polymerization decreased with the gamma-ray treatment, and the crystallinities and orientation angles were almost independent of the gamma-ray treatment. The thermal conductivities of the ramie fibers with and without gamma-ray treatment, decreased with decreasing temperature. The thermal diffusivities of the ramie fibers and those irradiated by gamma rays were almost constant from 250 to 100 K, increased slightly with the temperature decreasing from 100 to 50 K, and increased rapidly with I he temperature decreasing below 50 K. The thermal conductivity and thermal diffusivity of the ramie fibers decreased with the gamma-ray treatment. The mean free path of the phonon in the ramie fibers was reduced by the gamma-ray treatment. This decrease of the thermal diffusivity and thermal conductivity was explained by the reduction of the mean free path of the phonon by molecular chain scission with gamma rays. (c) 2006 Wiley Periodicals, Inc.