Abstract: The solidification microstructure of the in-situ synthesized Al3Zr/A356 composites by the direct melt reaction method in the A356-K2ZrF6 system under rapid solidification condition was investigated by X-ray diffraction,transmission electron microscope and scanning electron microscope.The results indicate that the Al3Zr particles synthesized under rapid solidification of single roller melt spinning was fine and in the shape of rectangle,the size was about 3-4 μm,which was 1-2 μm shorter than that of the particle under the conventional metal mold solidification.And compared to the conventional metal mold solidification,the reinforced particles under rapid solidification became easier to be captured by the dendritics in the rim of the solidification,and the particles distributed more uniform.Its primary crystal α-Al in the composite substrate A356 alloy become refiner,eutectic silicon existed in the forms of granulate and small bulk,which uniformly distributed in the substrate.