Abstract: Streptococcus thermophilus Q4 and Lactobacillus bulgaricus G5 which isolated and screened from Hezuo pastoral areas were mixed to ferment yogurt. Choosing factors and level by single factor experiment, optimized the fermentation condition of yogurt culture in accordance with the Box-Behnken experiment design. The results showed that:the best quality of the yogurt would be obtained when the fermentation temperature of 41.2℃, the addition of suger of 5.2%, inoculation rate of 3% and mixing ratio of 1∶1. In this case, the predict sensory evaluation score was 88.52 and the result of verification test of it was 89.67±1.53.