Surfactant-free, H-2-reduced Pt NPs were successfully fabricated on the surface of Fe3O4. Characterizations disclosed that Pt NPs (3.1 nm on average) dispersed evenly on Fe3O4. This catalyst is extremely active and selective for hydrogenation of chloronitrobenzenes, convenient, and suitable for cyclic utilization. The study showed that the activity of Pt depended mainly on its particle size and that the Fe3O4 support is most favorable for this reaction.