Abstract(#br)Magnesium (Mg) reinforced recycled-polystyrene (PS) matrix composites were fabricated through solution blending followed by compaction under different pressures (i.e., 6 MPa, 10 MPa, 60 MPa, and 100 MPa). The hardness of the composites initially increased and then decreased with the increase of the Mg weight fraction from 0 wt.% to 83.3 wt.%. The composite with 80 wt.% Mg exhibited the maximum hardness, which increased by 150% compared with that of PS. For each Mg weight fraction, there was an optimal compaction pressure that led to the highest hardness. Theoretical predictions of the hardness agreed with the experimental values through adjusting the reinforcement efficiency.