Abstract: To promote the independent research and development of domestic dairy starter，Streptococcus ther-mophilus H2 was isolated from Inner Mongolia traditional fermented milk． The composition of culture medium was optimized by response surface methodology，and the probiotic characteristics in vitro of strain H2 were evaluated by in vitro analysis． The results show that，the number of viable bacteria can reach 4. 20 ×10⁹ CFU/mL，37. 8 times larger than that before optimization，when the nitrogen source is 45.25g/L yeast extract，growth factor is 169. 5mg/L cysteine，buffer system supplement is 0. 705 g/L calcium carbonate，and the culture temperature is 40 ℃，initial pH value is 5. 5，inoculum size is 3. 5%． Evaluation of the probiotic characteristics in vitro show that the survival rate of Streptococcus thermophilus H2 is 49. 65% when pH is 3 and 99. 62% when bile salt is 0. 3%． Many kinds of fermentation metabolites，such as L-lactate，formic acid，acetic acid，propionic acid，isobutyric acid and isovaleric acid，were detected by high performance liquid chromatography (HPLC) in fermentation supernatant of H2． The concentration of L-lactate is up to 12. 64mmol/L，and D-lactate is not detected． The supernatant of H2 has scaven-ging effect on three free radicals，of which the scavenging rate of 1，1-diphenyl-2-trinitrophenylhydrazine (DPPH) was the highest，reaching 94. 06%．

Key words: Streptococcus thermophilus, culture medium optimization, response surface methodology, prebiotic properties