To improve the fermentation performance of Lactobacillus helveticus, Streptococcus thermophilus as a potential starter culture for functional fermented milk was utilized together with L. helveticus to produce fermented milk. As results, the mixed culture fermentation shortened the curding time by 3.5 h, increased yogurt viscosity by 1.8 times and improved whey syneresis and texture properties as compared to L. helveticus alone. Interestingly, S. thermophilus weakened the postacidification of fermented milk containing L. helveticus, maintaining a pH value greater than 4.2 for 20 d of storage. To account for this finding, the number of living bacterial cells, lactose consumption and the activity of glutamate decarboxylase and β-galactosidase were measured. It was found that the activity of glutamate decarboxylase and β-galactosidase of the fermented milk containing the two strains was lower, resulting in a lower lactose consumption and consequently weakening yogurt postacidification. This study concluded that co-fermentation with S. thermophilus was an effective means to improve the fermentation performance of Lactobacillus helveticus.