This experiment was conducted to study the relationship between performance and bacterial population in upflow anaerobic filter by improved Hungate technique. The results obtained showed as follows: (1) There existed correlation closely between the bacterial population, COD and volatile fatty acids removal; (2) The bacterial population was affected to a small extent by the change of organic loading rate in short time and suspending performance; (3) Successful performance of the reactor depended on the microbes' composition and their coordinated metabolism.