Abstract: The effect of amounts of yeast extract in fermentation medium on growth and metabolism of α- hemolytic streptococcus were studied. The results showed that: the number of colonies of α- hemolytic streptococcus increased obviously with the amount of yeast extract when the amount of yeast extract was from 3g / L to 7g / L,the most number of colonies was 1.9 × 109,2.5 times as high as the control group 7.6 × 108. The lag phase ofα- hemolytic streptococcus reduced from 8h to less than 5h. The logarithmic growth rate increased from 2.83 to11.02. And the stationary phase reduced from more than 27 h to 23 h.When the yeast extract content was more than7 g / L,those rules were no longer apparent. At the same time,the yeast extract made bacteria volume increase rapidly ahead of time for 2~4h.The largest specific growth rate was 0.38 at fermentation time 4h,and the maximum yield of mannan peptide was 1.04 g / L at fermentation time 27 h when the yeast extract content was 7g / L.