Abstarct: The bio-environmental problems made by burial of sewage sludge and the destructive effects created by disposal of this dangerous poisoning substance, caused attracting too much attention. In the other hand, increasing population and, consequently, increasing the volume of sewage, demands more construction of wastewater treatment plants, hence, the volume of sewage sludge is also growing. Therefore, establishing a proper cycle for application and use of this material, which is a waste substance of wastewater treatment plant, is a very convenient way to reduce the bio-environmental risks of burying it. One of the conventional methods of sludge disposal, especially in areas that are faced with scarcity of land, is burning it. The main problem of this method is how to dispose the ash left after burning the sludge. Moreover due to vast territory of collapsible soil and the compulsion of building dams, watering canals and other technical structures in those regions and also considering the growth of urbanizations and the necessity of expansion of some big cities over that kind of soil, inspection of the stability and improvement of collapsible soil is even more essential. The objective of this project is analyzing the effect of sewage sludge ash on collapsible soil sheer strength parameter. For this goal after obtaining the required sewage sludge from Tehran, Ikbatan, wastewater treatment plant and collecting collapsible soil, the use of sewage sludge ash as an additive was set on the agenda.After the primary assessments, sheer strength parameter of under investigation soil has been tested and determined without additive and with additive in different weight percentages, including 2%, 4%, 6% and 8%.The final results show that increasing the percentage of sewage sludge ash as an additive will also increase soil cohesion and decrease the internal friction angle ɸ. The effect of time is also investigated in this project which leads to this result: with increasing time of the soil treatment with sludge ash, like before, cohesion increases and the internal friction angle.At last, it can be concluded that considering the Mohr-Coulomb line in diverse conditions, with increasing this additive sheer strength increases for the small quantities of stress and decreases for the larger ones.