Abstract: Interleukin 21 (IL-21) is a kind of pleiotropic cytokine, mainly through its specific receptor (IL-21R) to exercise immunomodulatory function, including promoting T cells proliferation, regulating B cells differentiation and enhance the cytotoxicity of natural killer cells (NK), and play an important role in the immune response to pathogen infection. In order to study the role of IL-21 and its receptor IL-21R in the immune response of fish against pathogenic bacteria, homologs of IL-21 and IL-21R in Oreochromis niloticus were identified, and their roles in bacterial infection and the regulation of inflammatory response were investigated. The open reading frame of OnIL-21 and OnIL-21R are 420 bp and 1548 bp, encoding 139 and 515 amino acids, respectively. The deduced OnIL-21 is a secreted protein containing an Ig-like domain. OnIL-21R is a transmembrane protein, contains two conserved cysteine residues, a FNIII domain and a highly conserved WSXWS motif of the type I cytokine receptor family. Both OnIL-21 and OnIL-21R contained multiple conserved phosphorylation sites and glycosylation sites. Expression analysis indicate that the OnIL-21 exhibited constitutive expression in the examined tissues, with the highest expression in skin. OnIL-21R was also widely expressed in multiple tissues, with the highest expression in gill and spleen. In addition, the OnIL-21 and OnIL-21R expressions are significantly up-regulated in spleen and anterior kidney following challenges of Streptococcus agalactiae and Aeromonas hydrophila in vivo and in vitro, and maintained high expression level within 72 h, which suggested that OnIL-21 and OnIL-21R may get involved in host defense against bacterial infection. Further, after being stimulated with recombinant protein (r)OnIL-21 in vitro, the expressions of OnIL-21R and inflammation-related cytokines such as interleukin-1β (IL-1β), tumor necrosis factor α (TNF-α), interferon γ (IFN-γ), interleukin 6 (IL-6) and interleukin 10 (IL-10) were up-regulated significantly in spleen lymphocytes, which indicated that OnIL-21 and OnIL-21R may play an important role in immune response. Taken together, the molecular biological characteristics of OnIL-21 and its receptor OnIL-21R were described systematically. It also preliminarily clarified that OnIL-21 and OnIL-21R were likely involved in host defense against bacterial infection and may have an important effect on the inflammation response of O. niloticus, which may provide an important reference for further exploring the mechanism and signaling pathway of IL-21 regulating the immune response of the inflammatory responses through its specific receptor IL-21R.