The world is moving towards sustainability to conserve natural resources. Many environmentally harmful materials produced by different industries that cannot be recycled can be exploited as a substitute for natural resources. The presence of steel slag in millions of tons around the world has become a negative impact on the environment. In the effects of climate change, there is a need to move towards sustainability. The use of steel slag which is produced by the steel factories located in the country will reduce the depletion of natural resources, at the same time, a place can be found to dispose of them and reduce environmental damage and burial costs. The main objective of this research is to specify the optimum mix of natural coarse aggregate and coarse steel slag in the subbase layer. California Bearing Ratio and Aggregate Impact Value experiments are the main parameters that will be used to evaluate each mix. As a result, it turns out that the use of 19% of steel slag will meet the required efficiency for the paving subbase layer.