The inorganic constituents in flay ash such as Ca, Mg, Al and Si were extracted by water and 0.5N-Acetic acid, and changes of the physical properties of the fly ash-treated soils were examined to find out the effect of fly ash on the chemical and physical properties of the soils. The dominant day minerals of fly ash were quartz and mullite. More inorganic constituents were extracted from the fly ash by acetic acid than by water. Si and A1 in fly ash were hardly extracted by water. Addition of fly ash to soil below 10%(W/W) caused improvement in the water permeability and the field moisture capacity of the soil, but did not influence the shrinkage and hardness of the soil. Therefore, it was apparent that the effect of the fly ash on the chemical and physical properties of the soils greatly dependedon soil reaction, the organic acid contents, and the amount of fly ash used in treatment.