Abstract: 【Objective】To clone the cluster of differentiation 63 gene（CD63） of Nile tilapia（Oreochromis niloticus）and analyze its expression after stimulation by pathogenic bacteria, so as to explore the role of CD63 gene in immune response of Nile tilapia, which could provide theoretical basis for study of the immune function of CD63.【Method】CD63 gene of Nile tilapia was cloned, and the CD63 protein of Nile tilapia was analyzed by bioinformatics softwares, including physical and chemical properties, transmem brane domain prediction and phylogenetic analysis. The tissue expression characteristics of CD63 gene in Nile tilapia and its expression changes after infection with Streptococcus agalactiae were detected by real-time fluorescence quantitative PCR. 【Result】The CD63 gene of Nile tilapia had an open reading frame（ORF） length of 723 bp and encoded 240 amino acid residues. The molecular weight of CD63 protein of Nile tilapia was25.88 kD, the theoretical isoelectric point （pI） was 8.38, and the mean hydrophilic coefficient （GRAVY） was 0.745. It was a hydrophobic protein without signal peptide, containing 4 transmembrane domains. The similarity of CD63 amino acid sequence between Nile tilapia and other species such as Oreochromis aureus was high through comparative analysis.Phylogenetic analysis shows that Nile tilapia was closely related to Oreochromis aureus. CD63 gene of Nile tilapia was expressed in 10 tissues （brain, spleen, liver, skin, head kidney, gills, heart, intestines, blood and muscle）, with the highest relative expression in muscle, followed by blood and intestines, and the lowest relative expression in brain. The highest relative expression levels of CD63 gene in gills, heart, spleen, intestines, muscle and skin of Nile tilapia were significantly up-regulated at 3-96 h after infection with Streptococcus agalactiae compared with without infection （0 h）（P＜0.05, the same below）. At 96 h after infection, the relative expression of CD63 gene in brain, gills, head kidney,spleen, intestines, muscle and skin was significantly down-regulated compared with the highest value.【Conclusion】The expression of CD63 gene in tissues of Nile tilapia after infection with Streptococcus agalactiae is greatly changed, indicating that CD63 gene is involved in the immune response of Nile tilapia to pathogens.