Activated carbon fiber (ACF) filter developed for use in airborne radioiodine monitoring is described. To find a suitable ACF as an adsorbent for radioiodine, several kinds of ACF which have different specific surface area, pore diameter, etc. were examined for collection efficiency for methyl iodide. On the basis of these experimental results an ACF filter with 60mm diameter and 4mm thickness was made from 6wt% TEDA impregnated ACF by way of trial. The ACF filter is strongly made against mechanical impact and does not scatter the fibers. The pressure drop by ACF filter is 14mmAq at flow rate of 50l/mm, that is one fourteenth for activated charcoal filter paper. Collection efficiency of ACF filter for methyl iodide is 67%, that is 7 times higher than that of activated charcoal filter paper under the conditions; flow rate 5l/mm, sampling time 60min and relative humidity 90%. The applicabilities for airborne radioiodine monitoring in exhaust air of facilities were demonstrated.