To understand the influence to thermal conductivity by bridging in the polymer fibers, the thermal conductivity, and thermal diffusivity of ramie fiber and those bridged by formaldehyde (HCHO) using vapor-phase method (VP-HCHO treatment) were investigated in the lower temperature range. The thermal conductivities of ramie fiber with and without VP-HCHO treatments decreased with decreasing temperature. Thermal diffusivities of ramie fiber with and without VP-HCHO treatments were almost constant in the temperature range of 250-50 K, and increased by decreasing temperature below 50 K. Thermal conductivity and thermal diffusivity of ramie fiber decreased by VP-HCHO treatment. The crystallinities and orientation angles of ramie fibers with and without VP-HCHO treatment were measured using solid state NMR and X-ray diffraction. These were almost independent of VP-HCHO treatment. Although tensile modulus decreased slightly by VP-HCHO treatment, the decrease could not explain the decrease in thermal conductivity and diffusivity with decreasing sound velocity The decrease of the thermal diffusivity and thermal conductivity by VP-HCHO treatment suggested the possibility of the reduction of the mean free path of phonon by HCHO in VP-HCHO treated ramie fiber. (c) 2005 Wiley Periodicals, Inc.