Abstract: Four Lactobacillus delbrueckii subsp.bulgaricus strains were regarded as the research subject, studied the enzymes of metabolic pathway from lactose to lactic acid and H～+-ATPase, to find which enzyme was the most important one that results in postacidification of yogurt, and the reason why KLDS1.1006 ang KLDS 1.1011 had a weak postacidification character.The key enzymes of Lactobacillus delbrueckii subsp.bulgaricus played important roles in producing lactic acid wereβ-galactosidase and H～+-ATPase.Lactate dehydrogenase and protease showed no obvious correlation with postacidification.The enzyme activities ofβ-galactosidase and H～+-ATPase were weak, which was the fundamental reason why KLDS1.1006 and KLDS 1.1011 reflected weak postacidification.