Abstract: This study was aimed to investigate the structure and bioactivities of exopolysaccharides produced by Paenibacillus bovis sp.nov BD3526 in nitrogen-free solid culture medium. The exopolysaccharides were prepared, and then separated and purified by DEAE-sepharose fast flow and one main component was obtained ( F1) .Its molecular weight and molecular weight distribution was determined by gel permeation chromatography while its chemical structure was characterized by FT-IR and1 H NMR, indicating F1 was linear levan with ultrahigh molecular weight of 3.76 × 107 Da. It had good thermal stability, which was analyzed by thermogravimetric analysis ( TGA) and differential scanning calorimetry ( DSC) . DLS result indicated the particle size of its aggregates in aqueous solution was 163.2 nm while its conformation was characterized as compact sphere ( 50~ 100 nm) by TEM.Finally, its immunological activity was evaluated by an in vitro method and the result demonstrated F1 could significantly stimulate the proliferation of spleen cells, indicating it can be a potential natural immunomodulator. This levan could be an immunoenhancer to be used in functional food field.