Abstract: Cluster of Differentiation 81 (CD81) is a transmembrane protein also known as tetraspanins that plays an important role in cell growth, differentiation, signal conduction and adhesion. In this study, we cloned the CD81 gene of grass carp. The length of the cDNA was 1376 bp containing an 87 bp 5' untranslated region (UTR), a 581 bp 3' untranslated region (UTR), and a 708 bp open reading flame (ORF). The ORF had 8 exons and 7 introns, and encoded 235 amino acid residues. We applied real-time RT-PCR analysis to determine the pattern of CD81 gene transcription in different tissues of grass carp, and to measure the change in the transcript level after GCRV infection. The results showed that CD81 was expressed in all the tested tissues and was highest in head kidney. The transcript level was altered in gill, spleen, liver, intestines and head kidney after GCRV infection. Moreover, we used green fluorescent protein (GFP) to trace the subcellular expression of CD81, and the laser confocal electron microscopy revealed that grass carp CD81 was located on the cell membrane, which was the same in human.