A novel exopolysaccharide named Ebosin was produced by Streptomyces sp. 139, with medicinal activity. Its biosynthesis gene cluster (ste) has been previously identified. For the functional study of the ste7 gene in Ebosin biosynthesis, it was disrupted with a double crossover via homologous recombination. The monosaccharide composition of EPS-7m produced by the mutant strain Streptomyces sp. 139 ($ste7^-$) was found altered from that of Ebosin, with fucose decreasing remarkably. For biochemical characterization of Ste7, the ste7 gene was cloned and expressed in Escherichia coli BL21. With a continuous coupled spectrophotometric assay, Ste7 was demonstrated to have the ability of catalyzing the transfer of fucose specifically from GDP-$eta$-L-fucose to a fucose acceptor, the lipid carrier located in the cytoplasmic membrane of Streptomyces sp. 139 ($ste7^-$). Therefore, the ste7 gene has been identified to code for a fucosyltransferase, which plays an essential role in the formation of repeating sugars units during Ebosin biosynthesis.