Abstract: As a pivotal signaling mediator of Toll-like receptor (TLR) and interleukin (IL)-1 receptor (IL-1R) signaling cascades, the IL-1R-associated kinase 4 (IRAK4) is engaged in the activation of host immunity. To study its biological function in mandarin fish (Siniperca chuatsi), the expression profiling of the gene and its role in immune responses to the infection of infectious spleen and kidney necrosis virus (ISKNV) and S. chuatsi Rhabdovirus (SCRV) were investigated. Based on the unigene sequences of IRKA4 gene which was obtained from the transcriptomic results of S. chuatsi, specific primers were designed and the complete IRKA4 gene (named ScIRAK) was cloned and sequenced by SMART-RACE. Bioinformatics analysis demonstrated that the CDS of ScIRAK4 gene was 1389bp, encoding a 462 amino acid with an N-terminal death domain (DD) and a central protein kinase domain (PKc). The transcription profiles of IRAK4 in the tissues of S. chuatsi were characterized by fluorescent quantitative RT-PCR. The results showed that the highest mRNA expression was found in the liver (P<0.05), followed by that in the muscle, blood, brain and stomach (P<0.05). The transcriptions of the IRAK4 in the spleen of mandarin fish infected with ISKNV or SCRV were furtherly analyzed. The mRNA of ScIRAK4 was down-regulated in the mandarin fish infected with ISKNV and the lowest transcription was observed at 24 h post infection (P<0.01). By contrast, the mRNA of ScIRAK4 was up-regulated in the mandarin fish infected with SCRV and the higest transcription was observed at 12h post of the infection (P<0.01). These findings suggest that ScIRAK4 plays a crucial and different role in the immune responses of Mandarin fish infected with different viruses.