Magnesium has the smallest specific gravity in the structural metallic material applied to automotive parts. It is expected to be suitable light material to meet the needs of a vehicle weight reduction in recent years. However, magnesium alloy has low forgeability due to its crystal structure and low mechanical property at temperature over 373K compared with aluminum alloys. Therefore, magnesium application for automotive power-train part is quite few. On the other hand, Al-Ca series magnesium alloy has been developed by some researchers as low cost and heat resistant alloy. In addition, authors have developed the cast-forge process with semi solid injection molding process to improve the forgeability. The purpose of this research is to validate the feasibility of cast-forge process of Al-Ca series magnesium alloy applying the semi solid injection molding for preforming. Accordingly in this research, semi solid injection molded 4 mass%Al - 3mass%Ca magnesium alloy has been chosen because of a good heat resistant properties, and its forgeability and mechanical properties after forging has been studied. As a result, it is proved that 4 mass%Al-3mass%Ca magnesium alloy shows excellent forgeabilty and heat resistant compared to conventional magnesium alloys. It is expected that applying this new alloy to cast - forge process with semi solid injection molding and forging realizes low cost manufacturing process for automotive magnesium parts.