Semi-solid AZ91 magnesium alloys were produced using a cooling plate followed by quenching in the mold. The cast and T6 heat treated microstructures were microscopically characterized and subjected to hardness tests. It was found that the resultant microstructure in water quenched semi-solid specimen were finer than that in conventional permanent mold cast specimen. The hardness tests also showed that the hardness of quenched semi-solid cast specimen was higher than that of permanent mold cast specimen. This was attributed to large amount of precipitates which results from fine precipitation of eutectic phase at intergranular areas during the aging treatment after its complete dissolution.