The JAK2(Janus kinase 2) gene fragment sequence was cloned from liver of grass carp (Ctenopharyngodon idella) by RTPCR. The obtained cDNA fragment sequence was 671 bp, which encoded 223 amino acids. The amino acid sequence comparison results showed that the JAK2 amino acid of grass carp shared 70%-91% identity with other species. Phylogenetic tree of JAK2 generated by Neighbor Joining method suggested that all teleosts cluster together, and the grass carp JAK2 is clustered closely with that of zebrafish, mandarin fish and globefish. Then the expressions of different tissues including liver, muscle, brain, heart, spleen and mesentery fat tissue were analyzed by realtime fluorescence quantitative PCR. The results showed that the JAK2 gene of grass carp expressed in all tissues and highest in liver (P<<0.05）, lower in muscle, brain, spleen and fat tissue, lowest in heart（P<0.05）. This study will be helpful for further understanding the structure and function of JAK2 in C. idella.