B7-H3 is an immune co-stimulatory molecule of the B7 family that contains a set of immunoglobulin-V (IgV) and immunoglobulin-C (IgC) domains. To explore the evolutionary process of B7-H3 gene, we conducted a genome-wide structure and phylogenetic analysis of B7-H3 genes in currently sequenced genomes. On the basis of the available data, 17 mammalian B7-H3 genes were predicted to have tandemly duplicated 4Ig domains. The analysis of gene structure and phylogenesis reveal that these 4IgB7-H3 genes resulted from domain duplication. Nevertheless, no difference in function has been observed between the two isoforms. It is more likely that domain duplication in 4IgB7-H3 leads to functional redundancy.