In the landfills at where MSWI bottom ash is mainly disposed of, hardening of bottom ash after landfilled was frequently identified. If hardening of bottom ash occurs in landfill, hydraulic conductivity and air permeability decrease. Then, degradation and stabilization of waste are thought to be delayed as the result of inhibition of air intrusion and rain infiltration. In addition, hardening of a part of ash layer causes preferential flow and it results in partial wash out of mobile elements. If stabilization of ash landfill is defined to be complete elimination of pollutants, hardening is not preferable phenomena. However, if the stabilization is defined to be the minimization of emission of pollutants and stable capturing of mobile elements, hardening leads to early stabilization. In this study, to evaluate the effect of hardening on the wash out of easily mobile salts, evolution of hydraulic conductivity and change of leaching characteristics according to hardening were experimentally investigated.