Soft clay soil mixed with fly-ash, cement and hardening agent was studied by means of the laboratory test and field test such as UC(unconfined compression strength), CBR(California bearing ratio), PBT(plate bearing test). When the soft clay was mixed with hardening agent, the effect of shear strength is much larger than that of soil mixture with fly-ash and cement. Based on the results of laboratory test, the field plate load test was conducted in soft ground at the road construction site. The lotary which is equipment for mixing soil was applied in order to thoroughly mix the soft clay and hardening agent. Finally, applicability of this soil improvement technique was evaluated by the results of plate load test.