Abstract: The fresh yak milk was as material,calf rennet,microbial rennet and papain was used as milk-clotting enzymes to produce hard cheese. And it was studied that the milk-clotting enzymes types effected on yak milk hard cheese proteolysis during ripening. The results showed that there was a big difference in proteolysis of different milk-clotting enzymes yak milk hard cheese. To some degree,the TN,p H4.6-SN/TN,12% TCA-N/TN,5% PTA-N/TN and FAA increased gradually with the ripening time. Protein nitrogen and casein nitrogen decreased sequentially decreased. Polypetide nitrogen increased continuously and then decreased.The proteolysis of microbial rennet yak milk hard cheese was significant(p<0.05) higher than the papain and calf rennet yak milk hard cheese.