A method to grow GeSn nanodots has been developed by magnetron sputtering using anodic aluminum oxide as template. With a high substrate temperature and a high deposition rate, flattened hill-like GeSn nanodots with high Sn content and high density have been successfully formed directly on Ge (001) and Si (001) substrates. The GeSn nanodots are polycrystalline on Si and monocrystalline on Ge without Sn segregation. High-resolution transmission electron microscopy observations revealed that GeSn nanodots formed on Ge had a perfect interface without misfit dislocations.