The frost layer with melting water was measured by using a neutron radiography in the defrosting process on the aluminum finned-tube heat exchanger under the forced-convection. The water deposition distributions were estimated quantitatively in every 5 seconds for 10 minutes of the defrosting period. The melting water behavior was investigated by comparing the water deposition distribution before defrosting. The initial stage of the defrosting was referred to as the melting progressing period, in which the melting water penetrated into the water permeation layer in the frost thickness direction. The melting water penetration into the spanwise direction of the fins was not observed by the neutron radiography. After 180sec from starting the defrosting process, the frost at the fin front edge and the surrounding the tubes were melted, then, the water permeation layer was saturated by the melting water. The neutron radiography images indicated the melting water penetration into the spanwise direction of the fins. The total migration volume of the melting water into the spanwise direction was about 30% of the total frost deposition.