The purpose of the present study was to examine the influence of muscle cooling on the passive mechanical properties of the human gastrocnemius muscle (GAS) in vivo. In a thermoneutral (a room temperature of 18-23°C) and a local cooling (placing the right lower leg into cold water with a temperature of 5-8°C for 60 min) conditions, the change in passive plantarflexion force (F), which is produced only by the GAS length change, was taken in five subjects during passive knee extension from 90° to 0° with a constant ankle joint angle of 10° dorsiflexion. To evaluate an elastic component of the passive plantarflexion force of GAS, the subject held full knee extended position for 1 min (i.e. relaxation period). Skin and muscle temperature of GAS were also measured using a core temperature thermistor. The peak value of F (Fve) that was measured at the end of the knee extension phase, the decrease of F (ΔF) during the relaxation period, and the F at the end of the relaxation period (Fe) were measured in the two conditions. Muscle cooling decreased the skin and muscle temperature by 6.7 ± 1.1°C and 8.1 ± 2.5°C, respectively. Fve increased by 24% ± 22% by muscle cooling. ΔF in the thermoneutral and local cooling conditions were 11.5 ± 4.9 N and 12.5 ± 2.9 N, respectively. Fe increased by 28% ± 21% by muscle cooling. These results suggested that muscle cooling increased an elastic component of the passive force arisen from GAS and seemed not to affect its viscous component.