Now a day the world is seeking an alternative material to substitute natural resources needed for concrete production. It is essential to investigate the feasibility of using by-product or waste materials as aggregates to maintain the concrete construction industry. Several types of waste materials are currently used in concrete. One of the potential resources of waste material used in concrete is steel slag. It is known that Steel slag is the most significant solid waste generated by Steel Company, replacing natural aggregate with steel slag aggregate will have a significant environmental and economic impact to the country. In the first part of this thesis, the background of the study and the extent of the problem were discussed. A review of relevant literature was done to study previous works in the research topic. The research was carried out by conducting tests on the raw materials to determine their properties and suitability for the experiment. Concrete mix designs are prepared using the ACI method. There are 30 cubes and 10 beams for each fine and coarse aggregate replacement. The specimens were produced with percentage replacements of the coarse aggregate by 25%, 50%, 75% and 100 % of steel slag aggregate and fine aggregate by 15%, 30%, 45% and 60% of steel slag aggregate. Moreover, a control mix with no replacement of the coarse and fine aggregate was prepared to make a comparative analysis. The prepared samples consist of concrete cubes and beams. The results showed that at 50% replacement of the coarse aggregate by steel slag achieved a higher compressive strength for 7 and 28 days, whereas the 30 % replacement of fine aggregate by steel slag in the concrete have higher compressive strength for 7 and 28 days. Similarly, at 50% replacement of the coarse aggregate by steel slag achieved a higher flexural strength for 28 days, whereas the 45 % replacement of fine aggregate by steel slag in the concrete have flexural strength 28 days. Depending on the test result, it is recommended that the chemical reaction of each element and the optimum content will be investigate.