Today the attention of countries to the principles of sustainable development has led to increasing attention to environmental protection. Consequently, the processes and technologies are designed to come into operation in a way that minimizes waste and pollution. On the other hand, the maximum saving of energy and the resources are met. Slag, inevitably is a substantial part of the products that will arise following the production of iron and steel. Slag accumulation has a significant impact on the environment, including groundwater pollution. The great slag depots occupies a considerable area of ?? iron and steel factories, to the extent that in some factories, the area occupied by slag is more than the surface area of ??the product area, which is important from the economic point of view. On the other hand, slag physical properties, have made this aggregate suitable for use in road construction industry and much research has been done in this area. Unfortunately, many of these aggregates are not used in our country and mainly regarded as a waste material. In this study, we tried to improve some physical properties of slag, so that the aggregate layer can be widely used in railway sub-ballast. Field assessments have shown that the aggregate slag (slag produced in Mobarakeh Steel Complex) does not show enough shear strength for use in the sub-ballast and does not compact well. In this regard, the effect of Gilsonite, which is also named natural or mineral bitumen, was studied on shear strength of slag sub-ballast layers. In this study, in addition to slag materials quality control testing, California Bearing Ratio (CBR) test and direct shear test was conducted to evaluate the effect of Gilsonite on the shear strength, that positive results were gained. In these experiments, the maximum shear strength and CBR number was achieved in a mixing ratio of 2% Gilsonite. Keywords : Slag, Railway Sub-ballast, Gilsonite, Environmental Protection