Abstract: Bacillus subtilis B26 with antifungal activity against wood sapstain was isolated from the soil samples of Betula platyphylla Suk. Cells of Bacillus subtilis B26, as an original strain were mutated by UV irradiation to improve its antifungal activity. The mutant B2610 was obtained due to enhanced activity in inhibiting Trichoderna viride 7258, Ceratocystis fimbriata 8430, and Alternaria alternata 5173. Compared to the performance of strain B26, the antifungal activity of mutant B2610 was promoted to 60 ％ for T. viride 7258, 46 ％ for C. fimbriata 8430, 25 ％ for A. alternata 5173, respectively. Successive subculturing of B2610 for 10 generations did not cause its antifungal activity loss. The antifungal activity of the supernatant, fermentation broth and the broth after treated with heating were also tested. The antifungal activity of fermentation broth was slightly higher than those of the supernatant; the fermentation broth lost antifungal activity after heating. The results indicated that the antifungal activity seems to be related with the production of active metabolites,and the active metabolites could be proteins. The successful control of sapstain can be obtained by the pretreatment of Bacillus subtilis B2610 before the attack of sapstain. The results indicated that Bacillus subtilis B2610 could be a potent organism for biocontrol agent in the field of wood protection.