Abstract: Preliminary investigation on mass mortality of young fish Trachinotus ovatus cultured on sea cage in Yangjiang was carried out. The symptom of the sick fish was as follows: the skin was nigrescent, swam twisty or circularly, responded slowly and anorexia. There was no infection of parasites or bacteria. Under electric microscope there were virus particles about 30 nm in size. We primarily predicted it was infected by nervous necrosis virus.