The effect of fly ash on the strength and deformation characteristics of Bangkok clay is discussed in the paper. Lime or cement has been added as secondary constituents to further enhance self-hardening of the blended mix. For example, unconfined compression tests reveal that after two weeks of curing, a 18 0.000000ly ash and a 5 0me treated Bangkok Clay attains a compressive strength 2-3 times greater than that of the natural clay. However, if lime is replaced by cement, the initial rate of strength development increases significantly. Excessive fly ash contents (greater than 25%) cause tensile splitting of unconfined specimens. The compressibility of fly ashcement treated soil is considerably less than that of the natural clay. As the fly ash content exceeds 10 0.000000or a constant cement content of 5%, the increase of the equivalent yield pressure becomes significant. The reduction in compressibility is also associated with a corresponding increase in the coefficient of consolidation. The triaxial tests confirmed that fly ash-lime and fly ashcement treated specimens indicate a much higher sheafing resistance by improving the apparent friction angle, although the effect on the cohesion intercept is marginal. The stressstrain response changes from a normally consolidated state to an over-consolidated state as a function of the fly ash content. The findings of this study present a rational basis for the development of appropriate constitutive models for chemically modified soils.