Light weight concrete can be defined as a type of concrete which includes and expanding agent in that it increases the volume of the mixture while giving additional qualities such as inability and lessened the dead weight. The usage of Autoclaved Aerated Concrete blocks gives a prospective solution to building construction industry along with environmental preservation. Autoclaved Aerated Concrete (AAC) is versatile light weight concrete and they are generally used as blocks. The study of AAC replacing natural sand by fly ash is investigated. Design AAC mix having mix proportion 1:3 with water cement ratio of 0.6 in these specimens using with lime, the sand is constantly used in the specimens. The aluminum powder is used at the range of0.25, 0.5, 0.75, and 1gms of total weight of concrete. The properties of the mortar such as density, water absorption and compression strength for 24 hrs stream curing were determined. AAC is produced out of a mix of pulverized fly ash (PFA), lime, cement, gypsum, water and aluminum and is hardened by steam curing in autoclaves. As a result of its excellent properties, AAC is used in many building constructions, for example in residential homes, commercial and industrial buildings, schools, hospitals, hotels and many other applications. The construction material AAC contains 60% to 85% air by volume.