Waste materials, such as steel slag, which was a by-product generated during the production of steel and recycled concrete fine aggregate rubble, which is produced by demolition activities, are dumped in landfill and have a negative impact on the environment. They have become such attributes for possible replacements for natural fine aggregates in the production of mortar. In the present work, Steel Slag (SS) and Recycled Concrete Fine Aggregate (RCFA) were physically and chemically characterized. Thus, in this study recycled concrete fine aggregate from mortar cubes at a laboratory and steel slag in Addis Ababa Bishoftu steel industry factory were used to check the suitability of RCFA and SS for mortar production as a partial replacement of natural sand. A mix of mortar was prepared with 0%, 10%, 20%, 30%, 40%, and 50% replacement of natural sand by RCFA and SS. 384 (5x5x5) cm cubes were prepared and the suitability of produced mortar was checked by using flow table test, compressive strength ,sulfate attack, ultrasonic pulse velocity and water absorption test. Here the 0% replacement mortar was used as a control mix. The compressive strength of mortar was tested at the ages of a 3 rd , 7 th , 28 th and 56 th days and the result shows that, the compressive strength of mortar decreases as the replacement ratio of RCFA and SS increases. The highest compressive strength was obtained at 40% (20% steel slag and 20% recycled concrete fine aggregate) replacement. The outcome of the research suggests that using alternative materials for mortar production was very essential in the present construction scenario for sustainability and the preservation of natural resources. Keywords:- steel slag, recycled concrete fine aggregate, compressive strength, sulfate attack, UPV & water absorption.