Abstract: Natto is a healthy food but is not easily accepted by people because of its ammonia smell. In order to improve the flavor and taste of natto and reduce the beany taste, co-fermentation by Kluyveromyces marxianus and Bacillus natto was used in this study. This study used the sensory score and nattokinase activity as indicators, optimized the conditions of natto with compound fermentation by response surface methodology. The research results showed that the optimal conditions for the cofermentation of Kluyveromyces marxianus and Bacillus natto were strain ratio at 1∶1, fermentation temperature 36 ℃, and fermentation time 96 h with 3 005 IU·mL^-1 of the nattokinase activity. The natto scored 93 point with the highest taste and flavor under these conditions. In conclusion, the studty showed that the co-fermentation of Kluyveromyces marxianus and Bacillus natto could improve the flavor of natto.