Most of electricity is generated by thermal power plants in Iran. Shahid Rajaee thermal power plant consists of four 250MW natural gas and fuel oil burning units that is located at 100 km west off Tehran. As a result of fuel oil burning in winter time, boilers air heaters are washed and cleaned frequently. The wastewater originating from air heater washing contains suspended solids and different dissolved heavy metals particularly V, Ni, Zn, Cr, Pb. Wastewater is then treated in effluent treatment plant by chemical precipitation followed by centrifugal dewatering of sludge. Waste from this process contains significant amounts of heavy metals and is considered as specific industrial wastes that should be more characterized under specific wastes act of Iran. For the first time in Iran, in the present investigation seven composite samples from air heater washing wastewater treatment dewatered sludge were subjected to chemical composition, mineralogy and leaching characteristics studies to better understand the physical and chemical characteristics of this kind of wastes. Regarding fuel oil constituents and environmental impacts of metals the most likely pollutants which would be of concern in this study were heavy metals (Cd, Co, Cr, Mn, Ni, Pb, Zn and V). Results revealed that mean pH, wet and dry density and moisture content of the waste are 6.31, 1532 kg/m3, 1879 kg/m3 and 15.35 % respectively. Magnetite, SiO2, P2O5, CaO, Al2O3 and MgO were the main constituents of the waste with the weigh percent order of 68.88, 5.91, 3.39, 2.64, 2.59 and 1.76% respectively. Toxicity Characteristic Leaching Procedure (TCLP) test results for some heavy metals showed that mean elemental concentrations of Cd, Co, Cr, Mn, Ni, Pb, V and Zn in leachate were 0.06, 1.55, 5.49, 36.32, 209.10, 0.58, 314.06 and 24.84 mg/lit respectively. According to specific wastes act of Iran this waste can be classified as hazardous due to presence of heavy metals and leaching characteristics and should be disposed regarding hazardous waste disposal considerations.