The enzyme activity of α－Acetolactate Decaroboxylases (ALDC)from different microbes was studied, the results demonstrated that it was quite different among them. There were diversities of their enzyme reaction velocities. It was clear that the enzyme activity was affected by the pH of the enzyme reaction system, for example, the optimum pH of ALDC from Lactococcus lactis was 6.6, while for Aerobacter Aerogenes it was 5.8. Addition leucine,valine and isoleucine into enzyme reaction system obviously affected the enzyme activity of ALDC from different microbes.