Prophenoloxidase-activating factor(PPAF)plays a key role in prophenoloxidase activation system of crustacean's humoral immune system.In this study, the full-length cDNA sequence was cloned by reverse transcriptase polymerase chain reaction(RT-PCR)and rapid-amplification of cDNA ends(RACE), the expression patterns of PPAF gene in various tissues of normal shrimp and IHHNV-infected shrimp were determined by Quantitative real-time PCR.The results showed that the full-length cDNA sequence of PPAF was 1 986 bp, which contained a 21 bp 5'-UTR(untranslated region), 336 bp 3'-UTR(untranslated region), and the 1 629 bp open reading frame(ORF)encoded 542 amino acid.There was a serine protease domain between 269-517 amino acids residues of Litopenaeus vannamei PPAF, an ALPHA-2 macroglobulin functional site and a histamine enzyme active site were found within the 494-502 and 316-321 amino acids residues of Litopenaeus vannamei PPAF respectively.The expression of the gene in various tissues was analyzed by Real-time quantitative PCR, the result showed that the expression of the gene was higher in blood and gill than in heart, hepatopancreas, intestines, stomach and muscle of the normal shrimp or IHHNV-infected shrimp, but the expression of PPAF in various tissues of IHHNV-infected shrimp was lower than in same tissues of normal shrimp.The expression of PPAF in gill of Litopenaeus vannamei drastically reduced after infected with IHHNV, the expression levels reached a minimum within 3 hours, which was about 1/8 times as compared to the control group.After 3 hours shrimp infected with IHHNV, the expression of PPAF in gill increased gradually, and up to highest level after 48 hours.The results suggested IHHNV can protect them from the immunity of Prophenoloxidase by decreasing the expression of PPAF of shrimp when they infect shrimp.