Fly ash resulting from Municipal Incinerated Solid Wastes (MISW) can be potentially reused as cement replacement in concrete. However, treatment of these fly ashes is found to be pre-requisite for incorporation in concrete as partial substitution of cement. Previous researches indicate that the use of this type of fly ashes without treatment causes a drastic drop in the compressive strength due to expansion problem. The presence of considerable amount of aluminum is the origin of entrapped air produced when the water is added to fly ashes. This expansion process will continue during and after hardening of the paste leading to internal pressure and creating voids in the concrete matrix. In this research the process and the effect of treated MSWI fly ash are discussed and evaluated. Different methods were experimented using different chemical agents with water such as sodium hydroxide NaOH and Calcium Carbonate CaCO3. The physical and hardened cement paste properties were compared in terms of expansion and compressive strength for the different proposed treatments. It was found that MSWI fly ash collected from SICOMO plant in Lebanon has an attractive potential for addition as a mineral in the production of concrete provided that the risk of hydrogen entrapment bubbles in the fresh cement paste is prohibited. Key words: Fly Ash, Incineration, Municipal Solid Waste, Treatment, Waste Management