To evaluate the effects of different starter cultures and different fermentation substrates on the quality of fermented milk, the samples were prepared with Bifidobacterium animalis subsp. lactis (Ba) in co-cultures with traditional yogurt starter cultures (SL), and the acidification profile, viable bacteria count, water-holding capacity, proteolytic activity, rheological property, physical stability, microstructure, volatiles and sensory characteristic of fermented cow milk, fermented yak milk and fermented milk mixtures of cow to yak milk ratios of 1:1 were investigated. The results showed that for the same fermentation substrates, Ba-SL significantly improved the acidification property and viable bacteria count of fermented milk (P0.05). Ba-SL significantly enhanced the water-holding capacity of fermented cow milk and fermented mixed milk (P