In the present study, the influencing factors such as the intensity and the direction of gradient magnetic fields, the magnetic susceptibilities of non-magnetic metals on the structures are studied theoretically and experimentally. In the theoretical analyses, the influences of high gradient magnetic fields on nucleation and structures are investigated. In the experimental research, high gradient magnetic fields are imposed on paramagnetic material Al and diamagnetic one Sn during their solidification processes. Then the macro- and microstructures of these samples are examined and the influences of magnetic susceptibilities of metals, the intensity and the direction of high magnetic fields are analyzed in details. It is found that solidified structures could be refined when the magnetization force and gravity were in the same directions, while the solidified structures could be coarsened and the coarse dendrites grew along the direction of the imposed magnetic fields when the directions of these two forces were opposite. Those phenomena could be explained from the views of reduced gravity and elevated gravity effects caused by magnetization force and the convection suppression effect caused by high magnetic fields. The results indicate that high magnetic fields can be applied to control the solidified structures of metals and then improve the quality and the properties of materials for different purposes.