Microbial activities can be enhanced by organic rich leachate occurring in solid waste landfills, which possesses the potential to alter the barrier capacity of clay liners. Flexible-wall hydraulic conductivity tests are conducted to investigate the effect of the microbial activities on the hydraulic conductivity of Osaka marine clay used for clay liners of offshore solid waste landfill sites in Japan. Permeants with different redox potentials are employed to investigate the redox effect; and, permeants with high nutrients are used to check the effect of the microbial production in soil specimens. Test results indicate that there are no obvious changes in the free swell index, the liquid limit, or the hydraulic conductivity of the marine clay when a strong reducing agent is used. When nutrients are applied for the growth of microorganisms, however, a decrease in hydraulic conductivity, ranging from greater than two orders of magnitude to less than one order of magnitude, is observed. The formation of biofilm and anaerobic inorganic precipitation on the surface of the soil particles is considered to be responsible for this reduction in hydraulic conductivity. Test results reveal that microbial activities, enhanced by landfill leachate, may not cause an increase in the hydraulic conductivity of natural clay liners of offshore landfill sites.