Abstract: For the etiological researching of Duck's Cecum Enlargement and Necrosis Syndrome (DCENS), 38 20-days old ducks were fed with organ mixture from duck infected with DCENS, loach from the onset area and metacercariae isolated from loach respectively. Those fed with loachs and with 500 metacercariae showed typical symptoms of DCENS; but those fed with 100 metacercariae showed only mild symptoms and cecal lesion. The average number of Cyathocotyle caecumalis sp.nov isolated from the duck cecum of the group fed with loachs, 500 metacercariae and 100 metacercariae were 39, 61.7and 11.8, and eggs were checked out from all the three groups; while the group fed with organ mixture showed none symptom and Cyathocotyle caecumalis sp.nov detection. The results suggested that C. caecumalis sp.nov is the only pathogen of DSCEN. The loach from the paddy field of the onset area is the second intermediate host of C. caecumalis sp.nov.