Abstract: The article discusses the calculation method of the capillary equivalent radius of yarn made of moisture transmitting fibers through experiment.Using the capillary equivalent radius of yarn and other parameters,the wicking height can be theoretically forecasted,thereby the moisture transmitting property of fiber and yarn can be theoretically forecasted.The author slices five kinds of yarn and takes photos,and selects typical area from the cross section of the yarns,then the total area and the area of fibers in the typical area can be calculated.According to the above two areas,the area of capillaries is just the difference between the total area and the area of fibers.The number of the capillary can be gained via statistics.According to the area of capillaries,the number of capillary and the formula of circle,the equivalent radius of the five kinds of the yarns was gotten.The experiment result shows that among the five kinds of yarns,the equivalent radius of negative oxygen ion terylene filament is the longest,while the equivalent radius of Coolmax yarn is the shortest.