Abstract: A series of tests using ureolytic bacteria ATCC 11859 are conducted to reinforce the organic clay, with cementation reagent with different concentrations flowing to the soil under pressures. The reinforcement effect is comprehensively evaluated by comparing the changes of unconfined compressive strength, content of calcium carbonate, permeability, content of organic matter and concentration of ammonia and Ca²⁺ in the effluent. The results show that it is effective to treat the organic clay using pressure grouting. After treatment, the content of organic matter can be reduced by 1%~4%, the unconfined compression strength can increase by up to 370%, and the permeability coefficient can be reduced by about one order of magnitude. Under this bacteria condition (activity with 9.68 m M-urea hydrolysed•min^-1 and concentration with about 10⁸ cell/mL), the concentration of cementation reagent has a significant influence on the treatment effect. The unconfined compressive strength of the soil can be improved significantly by increasing the concentration of urea in 0.25 M-cementation reagent.