Monodisperse and size-tunable magnetic iron oxide nanoparticles (NPs) have been synthesized by thermal decomposition of an iron oleate complex at 310 ℃ in the presence of oleylamine and oleic acid. The diameters of the as-synthesized iron oxide NPs decrease with increasing concentrations of iron oleate complex and oleic acid/oleylamine. In addition, the size-dependent crystallinity and magnetic properties of iron oxide NPs are presented. It is found that larger iron oxide NPs have a higher degree of crystallinity and saturation magnetization. More importantly, various M–iron oxide heterostructures (M = Au, Ag, Pt, Pd) have been successfully fabricated by using the same synthesis procedure. The iron oxide NPs are grown over the pre-made metal seeds through a seed-mediated growth process. The physicochemical properties of Au–Fe₃O₄ heterostructures have been characterized by X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry and UV–vis spectroscopy. The as-synthesized Au–Fe₃O₄ heterostructures show a red-shift in surface plasmon resonance peak compared with Au NPs and similar magnetic properties to Fe₃O₄ NPs. The heterojunction effects present in such nanostructures offer the opportunity to tune the irphysicochemical properties. Therefore, this synthesis process can be regarded as an efficient way to fabricate a series of heterostructures for a variety of applications.