Objective To study the effect of nattokinse (NK) on the synthesis of poly(gamma-glutamic acid) (gamma-PGA) inBacillus subtilisnatto. Results gamma-PGA yield significantly decreased as NK was added in the original medium. With the increment of NK dosage, the yield decreased increasingly, but biomass increased instead of decreasing. The fact that cell density triggers the synthesis of gamma-PGA is a controversial issue. gamma-PGA yield and biomass closely correlate with addition time of NK. The later the addition of NK, the more gamma-PGA yield decreased but the more biomass increased. It is concluded that cell hunger is a key factor to trigger the transmission of the cell density signal, and NK may inhibit gamma-PGA synthesis by alleviating cell hunger. Besides, NK may reduce gamma-PGA yield by degrading extracellular gamma-PGA molecules. The study of adding L-glutamate of 0-20 g/L to the original medium showed that low concentration of L-glutamate (less than 5 g/L) could promote the synthesis of NK and gamma-PGA, and thus NK may inhibit gamma-PGA synthesis through strengthening substrate competition. Conclusions NK mainly inhibits gamma-PGA synthesis inBacillus subtilisnatto through alleviating cell starvation and strengthening substrate competition, and reduces gamma-PGA yield through degrading extracellular gamma-PGA molecules.