Olfaction is the important tool of sensing their surroundings to fish and may take part in spawning migration. Fish perform olfactory function by the means of the olfactory receptor proteins that olfactory receptor genes encode. The main olfactory receptor gene is the largest number of olfactory receptor families, which can identify water-soluble odor molecules. To find out the main olfactory receptor genes' differences of C. nasus between settlement population and migratory population, the MOR-2AK2 cDNA sequence of migratory C. nasus using rapid amplification of cDNA ends (RACE) was cloned. The sequencing results showed that the open reading frame was 972 bp in length, encoding 323 amino acid residues. The MOR-2AK2 protein had seven hydrophobic and alpha helix transmembrane structures, belonging to the G-protein-coupled receptor superfamily. Based on qRT-PCR among 10 tissues in migratory C. nasus, the result revealed that MOR-2AK2 was expressed in the olfactory rosettes and sex glands more highly than in others. Further analysis showed, the expression level in olfactory rosettes was 5 to 27 times higher than that in sex glands. In addition, there were differences in tissue expression quantity between male and female. The female olfactory rosette's expression level was 2 times that in male olfactory rosette. But the testis' expression level was 2 times that in ovary. Sequence analysis displayed that the 5 untranslated regions of MOR-2AK2 gene had microsatellite sequence, (GT)₅. The settlement population had a 14 bp more sequence (GTGTGTGTGTGTTT) than the migratory population. And the amino acid sequence similarity was just 84% between them. From the above, MOR-2AK2 gene not only took part in olfactory function, but also participated in gonad development or breeding migratory, and may be involved in the formation of settlement population.