Palladium (Pd) and palladium-platinum (Pd-Pt) colloidal nanoclusters catalysts have been prepared and immobilized on a magnetic support. Modification effects of magnetic support Fe3O4 and bimetallic structure on the catalysts have been studied by catalytic experiments and computational simulation. The Fe3O4 support benefits to catalytic selectivity of the nanoclusters in the selective hydrogenation of o-chloronitrobenzene to o-chloroaniline. X-ray photoelectron spectroscopy analyses indicate that the interaction between Fe3O4 and I'd atoms modifies catalytic performances of the nanoclusters. Computational simulation gives the atom distribution, in which Pd atoms are enriched on the surface of the Pd-Pt bimetallic cluster, while Pt atoms on the subsurface. It is found that the inner Pt atoms present beneficial modification on catalytic selectivity. In conclusion, the magnetic metal nanocluster catalysts provide good catalytic properties and easy isolation by magnetic field for their reuse. (c) 2008 Elsevier B.V. All rights reserved.