AbstractSteel slag is increasingly applied during road construction and maintenance processes due to its superior surface and mechanical properties such as hardness, rough surface, and wear resistance. In this paper, the effects of steel slag on mechanical performances in varied skeleton structures of asphalt mixtures are evaluated. The Marshall mix design method is modified, and a concept of nominal asphalt-aggregate ratio is proposed to match the asphalt mix design that includes steel slag. The aforementioned methods are then applied to the asphalt concrete (AC) and stone matrix asphalt (SMA) gradations. The performance tests are carried out to further evaluate the influence of the modified method on the mechanical properties for both AC and SMA gradations. Results show that the performance indexes of the steel slag mixture can satisfy specification requirements by using the modified Marshall mix design method; then, its effectiveness is verified. In contrast to the mixtures without using steel slag, the asphalt mixtures with steel slag show better rutting resistance of SMA gradation due to the characteristics of strong internal friction resistance and higher strength; the low-temperature crack resistance can also be improved, especially for the SMA gradation. The steel slag mixture does not show strong volume expansion and has good water stability. The skid resistance performance of the steel slag mixture is improved at a small level for both skeleton structures. The overall results indicate that the performance improvement effects of steel slag for asphalt mixture are obvious, and the modified Marshall design method can be used to guide the mix design for the application of steel slag in the asphalt mixture, which can significantly reduce the engineering cost and environmental effect for producing the asphalt mixture, promoting the sustainability of asphalt pavement.