Abstract: Yak milk hard cheese were made with three kinds of starter cultures which were mesophilic starter culture, thermophilic starter culture and mixed starter culture ( mesophilic ∶ thermophilic = 1 ∶ 1) .The influence of proteolysis and ACP on quality of yak milk hard cheese were investigated during 1¹80 days maturation.The results showed that the main ACP came from starter culture during yak milk hard cheese, protein degradation of cheese were significant effected by ACP.ACP and PPN showed strong positive correlation ( r = 0.720) , which had a strong negative correlation with the CN and PN. Three kinds of cheese stayed in a stable state during 60 ~ 120 days maturation.Different types of starter cultures were difference to degrade milk protein.The quality of cheese was affected by protein degradation.Mesophilic starter culture was the weakest for protein degradation of cheese, the cheese flavor was light.Thermophilic starter culture was the strongest for protein degradation of cheese, the cheese was bitter, but texture was better.Mixed starter culture was moderate for protein degradation of cheese, the cheese flavor was richness and texture was better.