Abstract: Compound Specific Isotope Analysis is an effective method to identify biodegradation of contaminants， which is the key for Monitored Natural Attenuation (MNA) of contaminated sites. In this work, the groundwater flow at a site contaminated by oil tank leakage was determined. Contaminants, microbial community structure, and electron acceptors in various zones at the site were characterized to elucidate biodegradation of benzene, toluene, ethyl benzene, xylene (BTEX). It was found that contaminant concentrations decreased in the following order: source zone, downstream source zone, plume downstream of the source zone, zone upstream of the source, and side plume. Similarities in the microbial community structure and in electron donors decreased in the same order. These observations can be explained by the decreasing hydraulic connection between different zones. Compound specific isotope analysis of toluene, xylene indicated that the extent of biodegradation decreased in the order of side plume, plume downstream of the source zone and downstream source zone. This order in extent of degradation in different zones was opposite to the order of degradation capacity derived based on consumption of electron acceptors, indicating that biodegradation at the site followed the Le Chatelier’s principle in accordance with general law of chemical reaction. This principle is that the higher the concentration of pollutants, the greater the amount of degradation, but the extent of degradation is relatively reduced.