Benzene is a toxic compound which is widely used as raw material in synthesis of styrene, nylon and in production of drugs, dyes, plastics etc. Because of toxicity, there is need to decontaminate the benzene. Benzene in waste water and biodegradation is very useful alternation to conventional clean – up methods. The success of this depends on mixed culture able to degrade in a changeable environment. The aim of work is to study the influence of pH of the medium and effect of inoculum size on benzene biodegradation by mixed culture. The effect of pH and inoculums size on biodegradation of benzene at initial concentration of 100 mg/L. pH is varied from 3.0 to 9.0. Benzene degradation was rapid at neutral pH (7.0) and decrease further increase or decrease pH. Benzene was degraded at different inoculum sizes. The degradation increases with inoculum size, but culture inoculated at 2.5 x 107 CFU/mL shows maximum degradation rate. This result is useful to understand the physiological and biochemical characters of mixed culture at changeable environmental condition.