Investigation on mass mortality of Trachinotus ovatus juveniles cultured in sea cage in Zhanjiang was carried out.The symptom of the sick fish was as follows:the skin was nigrescent,it swam twisty or circularly,responded slowly and anorexia.There was no infection of parasites or bacteria. The fish nervous necrosis virus was observed in the fish by PCR analysis. Two pairs of primers were designed based on the homogenus gene sequences from other fish nervous necrosis virus (NNV) submitted to GeneBank and the coat protein gene of Trachinotus ovatus nervous necrosis virus (PNNV) was cloned and sequenced. According to the analysis of similarity and phylogenelysis,the coat protein gene from PNNV had the highest identity of 99.2% to Epinephelus coioides nervous necrosis virus (ECNNV) at nucleotide level. Based on the result of molecular phylogenetic analysis,PNNV belonges to red-spotted grouper nervous necrosis virus (RGNNV) genotype. The cumulative mortality of all groups infected were 100% in artificial infection experiment by four kinds of infection. The infected samples were cloned and sequenced.These results suggested that the pathogen of mass mortality of Trachinotus ovatus juveniles cultured in sea cage in Zhanjiang was NNV.