Lost Foam Casting, pressurized casting, solidification, mechanical properties. Abstract. The paper presents the result of investigation on aluminum-silicon (LM6) alloy cast using pressurized lost foam casting process. The study investigated the effect of pressure and sand size on hardness and roughness of the casting produced. Air pressure of 1, 2, 3 and 4 bars was applied on the solidifying alloy poured in mould of sand sized 40-60(AFS). The roughness and hardness of the casting was measured using Hobson (Surtronic 3+) and Vickers hardness testing machine respectively and reported in the average of ten readings. For roughness measurement of the solidified castings, measurement was conducted in 25mm run whilst 11kgf was applied for Vickers hardness indentation test. The results show that using of finer sand size besides applying pressure during solidification of LM6 alloy has significant influence on roughness and hardness of the alloy. Increasing the applied pressure and used of fine sand size expedited heat transfer from the melt to the mould and surrounding thus decreasing solidification time. Consequently, reduced casting surface roughness as well as increased in hardness.