The feasibility of friction stir welding to joining of AC4CH semisolid cast aluminum alloy with 6061 aluminum alloy plates was investigated, including the influence of the joining condition on the mechanical properties of the joints. Excessive heat input conditions with high tool rotational speed and low welding speed result in formation of large burrs. Although the hardness of the stir zone is scarcely affected by the welding condition, the welding speed as low as 1mm/s makes heat-affected zones on both sides softer than the stir zone. Better mechanical properties, such as the tensile strength of approximately 85% of the AC4CH base metal and the elongation exceeding the basemetal, are obtained under low heat input conditions.