Slurry seal is a preventive maintenance method to preserve pavement. This research is focused on the effect of steel slag filler (SSF) as a natural filler (NF) substitute on the behaviour of slurry seal mixtures. Five different combinations containing 100% NF, 25% SSF + 75% NF, 50% SSF + 50% NF, 75% SSF + 25% NF, and 100% SSF based on the total filler weight were used. The samples were evaluated by the wet cohesion, wet track abrasion, loaded wheel-displacement, and loaded wheel-sand adhesion tests. For investigating the SSF impact on the performance characteristics of mixtures, an analysis of variance (ANOVA) was performed. The results indicated that mixtures with 75% SSF increase the bitumen cohesion by 45%, decrease the displacement by 50.8%, improve the abrasion resistance by 48.3%, and reduce the bleeding by 29.3%. ANOVA results showed that SSF has the greatest effect on increasing the adhesion and reducing the displacement in slurry seal.