The open reading frame of cellular nucleic acid-binding protein (CNBP) was cloned from Grass Carp (Ctenopharyngodon idella) cDNA library. The coding region encodes a 163-amino acid polypeptide with the highly conserved general structural organization of seven CCHC zinc finger domains, nuclear located signal and a RGG box which was highly conserved in fish. Compared with human and other vertebrates, the fifth glycine residue at the third zinc finger domain was substituted with histidine and 6-14 amino acid residues were absent between the first and the second zinc finger in the Grass Carp CNBP. Nevertheless, the grass carp CNBP was not in the positive Darwin selection (ω≤1) by adaptive evolution analysis. Therefore, new function was not generated by structural difference of CNBP. These indicated that CNBP was in the neutral selection.