X ray is able to penetrate bulk metallic materials with specific thickness (millimeter scale for heavy metal like iron or nickel based alloys and centimeter scale for light alloys like aluminum or magnesium) due to its high penetrability. Therefore, it can be used for materials imaging to obtain their two dimensional (2D) and three dimensional (3D) microstructures. Comparing to traditional characterization methods, Xray imaging is nondestructive and can be applied for in situ and realtime observation of microstructure evolution during material processing under certain space and time resolution. The third generation of synchrotron Xray source can generate high flux, high energy, high resolution and high coherence Xray beam with which we can achieve precise and fast realtime characterization and measurement of microstructure evolution of metallic alloy from macroscale (centimeter) to microscale (micron or submicron and nanometer). In this paper, the principle theory of Xray imaging method and its application in metal solidification microstructure static 3D characterization and 2D and 4D (3D plus time) in situ observation of structure evolution are reviewed. Finally, the potential applications of Xray imaging in solidification of metallic materials are also expected.