Abstract: The effect of a microbial mixture of Candida tropicalis and Streptococcus faecalis on the remediation of petroleum-contaminated soil was evaluated in a laboratory. The changes of dehydrogenase activity of soil and petroleum constituents were monitored during the bioremediation process. The results showed that the microbial mixture showed synergistic effects in the removal of petroleum hydrocarbons. In the mixed microorganisms treatment group, 29.9% of the total petroleum hydrocarbon (TPH) was removed after 32 days of incubation, which was 50% higher than that observed in the single microorganism-treatment groups. Higher dehydrogenase activity was observed in the mixed microorganisms treatment group, and a positive correlation was obtained between dehydrogenaseactivity and biodegradation efficiency. Within the early stage of treatment, low molecular weight alkanes were degraded significantly, while high molecular weight components (C15-C21) were metabolized beginning from 32 days.