Abstract: The magnetism of Pt900 jewellery has been studied by means of electron probe and X ray diffraction techniques. It is found that the Co is the cause of magnetism Pt900 materials. In the casting process of Pt900 materials, the strong magnetic materials, CoPt and CoPt₃ inter metallic compounds, which result in the magnetism of Pt900 materials, can be grown, and the Pt900 jewellery can be attracted by magnet.