Abstract: The changes of quality characteristics of yogurt made from yak milk and cow milk were analyzed through measuring the pH,total acidity,texture,diacetyl and acetaldehyde contents during the fermentation. During the whole fermentation period,total acidity of yak yogurt and cow yogurt showed an increase trend,while the pH decreased accordingly,and there were no significant differences between the two kinds of yogurt(p>0.05). The texture properties of the two kinds of yoghurt did not change during 0~3 h,and the hardness,consistency,cohesiveness and viscosity increased significantly during 3~11 h(p<0.05),thereafter these four texture properties changed unsignificantly(p>0.05). The texture properties of yak yogurt were higher than that of cow yogurt. The diacetyl and acetaldehyde contents of the two kinds of yogurt increased significantly during the pre-fermentation stage(p<0.05),and there was a certain fluctuation in the post-fermentation stage keeping at 7~11 mg/L. There were no obvious differences between yak yogurt and cow yogurt in diacetyl and acetaldehyde contents. Conclusion:For two yogurts made from yak milk and low milk,the change regularities of quality characteristics were same during the whole fermentation period,and there were no significant differences between the two kinds of yogurts in the tested indexes except for texture indexes.