The paper describes the results of stabilization tests of two types of loess clays obtained from road construction sites in the area of Moravia, which were treated with 5%, 10% and 15% additions of two fly ashes: siliceous one from a conventional boiler and calcareous one from combustion in a fluidized bed boiler. The obtained results showed that both investigated fly ashes have the potential to use them for soil treatment. The mechanical properties of soils depend on the grain size distribution and the clay fraction content. They affect mainly moisture content and load capacity. Fly ash from the power plant at the iron and steel plant in Třinec exhibits hydraulic properties, and its CBR capacity index was 500% after 28 days of maturing. The addition of this fly ash caused a reduction in the maximum bulk density of the soil backbone of the treated soils and increased shear strength, as well as influenced the control of volume changes.