Steel Slags are the by-product of iron and steel factories that usually tump in large landfills around these factories. Along the years, widespread researches have been done around the world to use this material in several fields. One of these fields is pavement engineering specially Asphalt Concrete (AC) that can yearly consume a large volume of slags. The studies show that AC containing steel slag have some disadvantages in spite of different privileges. One of the most important of them is the higher optimum bitumen content in comparision with conventional asphalt concrete because of higher effective surface. This concept increases the total cost of slag-asphalt concrete and decreases the tendency of investors for the use of such product. In this study, it is tried to decrease the optimum bitumen content of slag-asphalt concrete by reinforcing with polypropylene fibers. In this way, the optimized asphalt content was identified by using Marshall Method. Experimental studies are in two sections. The first section is the determination of optimum bitumen content in conventional and slag asphalt mixtures. It should be noticed that only coarse aggregates have been replaced by slag particles. Since the specific gravity of coarse and fine aggregates are different and the basic assumption in ordinary design of aggregate gradation is the specific gravity of material, so a modification on the aggregate curve have been done. In the second section, different percents of fibers(2, 4 and 6) by weight of bitumen with various lengths(6, 12 and 19 mm) have been added to asphalt mixtures. The results of experiments show that conventional asphalt concrete and asphalt mixtures containing slag particles have a 4.7 and 4 percent optimum bitumen content, respectively. Marshall stability of slag asphalt mixtures are is about 15 percent higher than conventional. Moreover, between different samples, slag bitumen mixtures containing 2 percent by weight of bitumen of 19 mm polypropylene fibers have the most appropriate performance. In this case, Marshall stability increases about 18.9 percent. Also, presence of fibers in slag-asphalt concrete causes a decrement in optimum bitumen content about 20 percent. At the end of study, an economical analysis have been done. It shows that the final cost of slag asphalt concrete is about 21.7 percent higher than conventional asphalt concrete. But in the case of fiber slag asphalt concrete the extreme cost is about 15.2 percent. Keywords: Steel Slag, Slag Asphalt, Polypropylene fibers, Asphalt Reinforcment