Abstract: Realoxifen hydrochloride is a new synthesis compound, used for the prevention and treatment of osteoporosis and other disease caused by osteoporosis. For further study on it, mass spectrometry techniques, such as electron ionization mass spectrometry (EIMS) and fast-atom bombardment mass spectrometry ( FAB(-) MS) can provide valuable information and was used for the elucidations of its structure and fragmentation pathway. The results shows the molecular ion peak m/z 510 M+HCl+H -, m/z 509 M+HCl -, m/z 473M - and m/z 472M-H - were detected by FAB(-) MS; m/z 473M + and m/z 474M+1 + detected by EIMS. And the molecular formula of realoxifen hydrochloride could be certified as C 28 H 27 NO 4S. Furthermore, the fragmentation pathway of raloxifen hydrochloride is also explained.