Abstract: The presence of chemical compounds has different effects on the rate of degradation of materials and on changing dominant microorganisms in the biological degradation processes. These effects are related to the type and concentration of compounds. This study was to evaluate the effect of different concentrations of glucose as a growth substrate on phenol removal efficiency and microbial flora in a serial combined biological system. Phenol an only substrate was injected to the system and then different concentrations of glucose (50, 250 and 500 mg L-1) were added. At the end, phenol removal efficiency and changing of dominant microorganisms in the separated combined biological system were evaluated. The study revealed that microbial flora were changed in various concentration of glucose and degradation of phenol was better in the presence of low amount of glucose (50 mg L-1) and decreased by increasing the glucose concentration. The dominant microorganisms in this research were Moraxella and E. coli. Presence the low concentration of glucose will increase the phenol removal and change the dominant microorganism in biological degradation processes.