Abstract: The quantitative effects of fermentation conditions on the yield of Streptococcus thermophilus( Q4)extracellular polysaccharide were optimized using response surface methodology( RSM).The EPS content as an index,fermentation conditions of EPS produced by Q4 was preliminary obtained by single factor experiment.Then significant factors were screened from the above the single factors according to the Placket- Burman( PB) design method and were optimized using Box- Behnken design.And the EPS content as the response values,obtaining the response surface figure,the optimal fermentation conditions on the yield of( Q4) extracellular polysaccharide were determined: initial p H6.8,fermentation time 33 h and fermentation temperature 41 ℃.Under the optimal conditions the corresponding response value for extracellular polysaccharide production was 268.42 mg / L,compared with the previous EPS content( 89.43 mg / L),which was increased 3 times.