We present a systemic investigation of a galvanic replacement technique in which active-metal nanoparticles are used as sacrificial seeds. We found that different nanostructures can be controllably synthesized by varying the type of more noble-metal ions and liquid medium. Specifically, nano-heterostructures of noble metal (Ag, Au) or Cu nanocrystals on active-metal (Mg, Zn) cores were obtained by the reaction of active-metal nanoparticles with more noble-metal ions in ethanol; Ag nanocrystal arrays were produced by the reaction of active-metal nanoparticles with Ag(+) ions in water; spongy Au nanospheres were generated by the reaction of active-metal nanoparticles with AuCl(4)(-) ions in water; and SnO(2) nanoparticles were prepared when Sn(2+) were used as the oxidant ions. The key factors determining the product morphology are shown to be the reactivity of the liquid medium and the nature of the oxidant-reductant couple, whereas Mg and Zn nanoparticles played similar roles in achieving various nanostructures. When microsized Mg and Zn particles were used as seeds in similar reactions, the products were mainly noble-metal dendrites. The new approach proposed in this study expands the capability of the conventional nanoscale galvanic replacement method and provides new avenues to various structures, which are expected to have many potential applications in catalysis, optoelectronics, and biomedicine.