In this paper,Streptococcus thermophilus from bringed in directed-vat-set was incubated with high cells population,and the apoptosis,growth curve,LDH activity,LD content of the bacteria were determined respectively during the incubation,The result of the experiment showed that apoptosis exists in Streptococcus thermophilus during the incubation,and is related to the rule of growth and acid making.