Abstract: The study aims to optimize the solid-fermentation conditions of Bacillus natto and determine the suitable solid- fermentation medium composition. The spore number was used as index. Effects of the fermentation conditions, such as a medium component, the ratio of substance to water, inoculation, initial pH value, medium loading, cultural time and so on, on spore number were studied by using solid- state fermentation method, and the optimal fermentation condition of Bacillus natto solid fermentation was determined by using response surface analysis. The results showed that the optimal fermentation conditions were as follows: wheat bran 58%, zeolite powder 34%, soybean cake powder 5.35%, glucose 2%, KH2PO4 0.5%, MgSO4 ·7H2O 0.15%, pH 7.56, the ratio of substance to water 1:1.1, 50.39 g/250 mL triangle flask, inoculation 4.16%, fermentation time 4 d. Under these conditions, the number of spore reached 6.48×1010 CFU/g.