In this research the effect of the reinforcement by fibers on Mechanical properties of polystyrene resin reinforced by aramid fibers was studied .The properties included impact strength , tensile strength Where the Mechanical properties were extracted for polystyrene resin before reinforcement by fibers, then we reinforced the resin by different weight percentage from aramid fibers (%15,%30, %45, %60)and studied the effect on the above mechanical properties. An Improvement was happened in these properties after reinforcement by fibers The value of mechanical properties will increase with increasing percentage of reinforcement . Impact strength increased from(8Kj/m2) to (160Kj/m2) and compressive strength from (37Mpa) to (328Mpa) for reinforcing percentages 0% and 70% respectively .