This paper is the basic study on caudal fin regeneration using killifish, an available teaching materials in secondary school science. The findings are concerring experiments on caudal fin regeneration of killifish us follows. 1) The regenerating speed changes with the amputation level or the time lapse after the amputation of the caudal fin of the killifish, MEDACA. 2) The blood coaguration generally occures near the amputation plane within 1 minute after the amputation. 3) The blood stream is observed in the regenerating area of the caudal fin about 3 days after the amputation. 4) Cuboidal cells in the epithelium arrange into one layer, and osteoblasts appear to migrate to the inside of the cuboidal cells and to form a fin ray dermal bone 2 to 3 days after the amputation of the caudal fin. The killifish, MEDACA, is very useful as an experimental animal for the study of the regeneration since the fin rays and the blood stream can be easily observed microscopically and the period from the amputation to the complete recovery is relatively short. Some teaching materials for the secondary school science are producible from the results of this fin ray regeneration. Additionally, the application of this experiments for the secondary school science is also considered.