Abstract: We isolated the bacterial strain B731, which is antagonistic against Alternaria solani causing tomato early blight, from the tomato rhizosphere soil with dual-culture method. The strain was identified as Bacillus subtilis according to its morphological, physiological, biochemical and molecular characteristics. The optimal conditions for B731 to produce antifungal substance were incubation of 40 mL of NB medium with initial pH 7.0 in a 250 mL flask at 28 ℃ with shaking at 120 r/min for 24—48 h. Under such conditions, B731 showed strong colonization ability both on the surface and in the rhizosphere soil of tomato plants. At 15 d after inoculation, the number of colonies was more than 2×10² cfu/cm² on the surface of tomato leaves or fruits, or 10⁴ cfu/g in the rizhosphere soil of tomato plants. The inhibition rates by the metabolites of B731 to spore germination and mycelial growth of A. solani were 89.72% and 82.62%, respectively. When the liquid culture was sprayed on the tomato leaves, the severity of tomato early blight was reduced by up to 81.83%.