In order to clarify the role of lactic acid bacteria and yeast in the yak yogurt quality formation, by using the yak milk powder as the raw material, the yak yogurt as the fermentation agent, and inhibiting the yeast and lactic acid bacteria in fermention, respectivly, the indexes, for the flavor, taste and texture, were determined, and compared in the ordinary fermentation, inhibition of lactic acid bacteria, inhibition of yeast fermentation yogurt. The results showed that lactic acid bacteria and yeast had a key role in the formation of yak yogurt quality. The lactic acid bacteria had a great contribution to acidity, texture, amino acid nitrogen, organic acid, V_B1, ketones and acid flavor compounds, and yeast has a greater contribution to ethanol, V_B2, alcohol and ester flavor compounds. Through exploiting the fermentation agents, mixed the lactic acid bacteria and yeast strains, it will help produce high quality yak yogurt.