The hemorrhage disease of grass carp Ctenopharyngodon idellus isan acute eisease caused by Reovirus which injure and damage blood capillaries ofthe viseccra and muscular tissur of fish body wall and bleed in varying degrees. Ob-servations under electron microscope indicated: (1) The morphological structures of diseased cell are abnormal. (2) The amount of organellis in diseased cells are lessthan that of the normal cells. (3) The demarcations among three layers of plasmale-mma structure are blurred and so far as to be disintegrated. (4) The numbers ofrough endoplasmio reticula, smooth endoplasmic reticula, Golgi complex and mito-chondria that concern metabolism decreased and their structures were seriously da-maged obviously.