Ground improvement is necessary in many flat areas and landfill sites in Japan because these areas have soft ground and are highly susceptible to serious damage such as long-term consolidation settlement and liquefaction. The deep mixing method (DMM) is an in-situ soil treatment in which native soils or fills are blended with cementitious and/or other materials. Ground treated by DMM has higher strength and lower compressibility than untreated ground. However, there are quality problems in this method due to a condition in which a mixture of soil and materials adheres to and rotates along with the stirring blades without performing efficient mixing. Therefore, our purpose is to improve quality of improved columns produced by the mechanical mixing method using vertical rotary shafts and mixing blades. In this study, five cases of small-scale model experiments were conducted with changing in the blade rotation number, the incident angle of agitating blades, and the agitating blade angle. Strength tests were conducted using unconsolidated samples at different depths to investigate strength distribution, and needle penetration tests were also conducted. From the results, the effects of the blade rotation number, the incident angle, and the agitating blade angle on improvement quality were discussed.