The monodispersed hydrophilic magnetic fluids with nanometric MxFe3-xO4 (M = Cu, Co, Ca and Ni) particles were prepared by sonochemical method. The substituted M amounts were analyzed with different x values by ICP-AES quantitatively. The excellent substitutability and magnetic property for Co, Ni was observed compared to those for Cu and Ca relatively. In particular, the applicability of Co was confirmed for novel radiotherapy. (C) 2007 Elsevier B.V. All rights reserved.