Abstract: The ability of acidification and viscosity production about 5 strains of Streptococcus thermophilus after fermented 8 h and 24 h in skimmed milk have been compared in this paper. Two strains( ST5 and ST9) were chosen to study the changes in p H,acidity,viscosity and amino nitrogen.It was showed the order of ability to produce acid was ST5 > ST,ST7 > ST > ST9 and to produce viscosity was ST5 > ST7 > ST > ST1 > ST9. So it could be seen that the ST5 fermented milk obtained the highest acidity and viscosity and ST9 was in the opposite. In further study,ST5 also presented a higher ability than ST9 at different fermentation time,while the content of reducing sugar and total sugar was lower and capacity to degrade proteins was weaker.In conclusion,the fermentation characteristics in skim milk were different form the strains of Streptococcus thermophilus and changed as fermented time went on.