There are many sites contaminated by chlorinated solvents all over Japan. Recently, monitored natural attenuation (MNA) has been developed to clean up those chemicals. MNA utilizes attenuating processes including biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization of contaminants. Natural attenuation of chlorinated solvents, such as tetrachloroethylene and trichloroethylene in contaminated groundwater in Takahata town, was studied. The correlation between natural attenuation behaviors was investigated in order to understand the mechanisms involved in natural attenuation process and to forecast future conditions. The degradation behaviors of chlorinated solvents were different between each well. The chemical components in the groundwater of the degradation with by-products showed lower redox potential, dissolved oxygen and higher heavy metals such as iron and manganese. These anaerobic reducing ambient is suitable for the bacteria concerned with the reductive dechlorination. Although in certain cases, vinyl chloride or ethylene can be observed. In this site, the higher degradation product was 1,2-cis-dichloroethylene while vinyl chloride and ethylene only appeared in a very low concentration. Those results suggest that only a small proportion of bacteria have the ability to degrade 1,2-cis-dichloroethylene to ethylene. To improve the biodegradation process toward the formation of ethylene, the effectiveness of different medium conditions was evaluated.