Abstract: In vitro antagonism of Beauveria bassiana, an entomopathogenic fungus, was tested against Fusarium oxysporium f. sp. vasinfecturn, pathogen of cotton wilt, and F. verticillioides, pathogen of cotton red rot. On dual culture, B. bassiana showed obvious inhibition on the growth of the two pathogens. Different doses of conidia had different inhibitory effects, with inhibition rate linearly correlated to inoculum, suggesting that the antagonism was associated with competition for nutrition and space. The inhibition on the two pathogens was both over 90% at inoculum of 6×10⁷ spores·mL^-1, slightly higher on F. verticillioide. The metabolite in SDY shake culture of B. bassiana also displayed inhibition, with inhibition rate linearly correlated to metabolite concentration, suggesting that the antagonism was also associated with antibiotic substances. Based on the comparison of inhibition rates, antibiosis was considered to contribute more than competition. In the analysis on the antibiotic metabolite, none of the lyophilized supernatant extracted by 4 different organic extractants showed obvious inhibition on the Fusarium spp., while addition of crude extract of either polysaccharide or protein significantly inhibited mycelium growth of the two Fusarium spp., suggesting that the main inhibitory substances are polysaccharide and protein with high molecular weight.