Abstract: Fos is one of member in activator protein 1（AP-1） family and played crucial functions in animals immune system. In this study, the c DNA sequence of Penaeus japonicus Fos gene（named PjFos） was obtained and analyzed. The full length c DNA was 1 955 bp, including 5’ untranslate region 95 bp, 3 untranslate region 348 bp,open reading frame（ORF） 1 512 bp which encoding 503 amino acid. Sequence analysis revealed that PjFos protein contained a basic leucine zipper（bZIP） domain and two nuclear localization signal（NLS） sequences. No transmembrane domain and signal peptide were predicted successfully. The result of protein advanced structure prediction showed that bZIP domain was α helix. Multiple sequence alignment analysis showed that PjFos had the highest homology with Fos of Penaeus vannamei（94%） and bZIP domain was conserved among most species. Phylogenetic tree analysis discovered that P. japonicus Fos was close to P. vanamei. PjFos was ubiquitously expressed in all tested tissues. The highest expression level was detected in the ganglion, followed by hepatopancreas and gill. Under the challenge of Vibrio harveyi, the expression of PjFos in gill was significantly up-regulated compared with the control group（P＜0.01）, reached the maximum at 24 h. These results indicated that PjFos may play a key role in innate immunity of P. japonicus.