The application of three different strains of Streptococcus thermophiles strains co-cultivation with Lactobacillus bulgaricus ND02 on gelation process during milk fermentations has been investigated using rheological technology. Meanwhile， to screen one Streptococcus thermophiles strain with excellent fermentation properties， viscosity， acidity and the number of viable cells were evaluated during fermentation and storage. The results showed that the viscosity and acidity between the three fermented milk samples was significantly different during fermentation and storage （P<0.05）， while the number of viable cells was higher than 1×106 CFU/mL. The three different strains of Streptococcus thermophilus had different effects on gelation process during milk fermentation. The Streptococcus thermophiles GW21-4 could improve the quality of yogurt and enhance gel structure. In summary， the Streptococcus thermophilus GW21-4 has the potential to be used as a starter culture for milk fermentation.