Wire-in-tube structures have previously been prepared using an electrospinning method by means of tuning hydrolysis/alcoholysis of a precursor solution. Nickel–zinc ferrite (Ni_0.5Zn_0.5Fe₂O₄) nanowire-in-nanotubes have been prepared as a demonstration. The detailed nanoscale characterization, formation process and magnetic properties of Ni_0.5Zn_0.5Fe₂O₄ nanowire-in-nanotubes has been studied comprehensively. The average diameters of the outer tubes and inner wires of Ni_0.5Zn_0.5Fe₂O₄ nanowire-in-nanotubes are around 120 nm and 42 nm, respectively. Each fully calcined individual nanowire-in-nanotube, either the outer-tube or the inner-wire, is composed of Ni_0.5Zn_0.5Fe₂O₄ monocrystallites stacked along the longitudinal direction with random orientation. The process of calcining electrospun polymer composite nanofibres can be viewed as a morphologically template nucleation and precursor diffusion process. This allows the nitrates precursor to diffuse toward the surface of the nanofibres while the oxides (decomposed from hydroxides and nitrates) products diffuse to the core region of the nanofibres; the amorphous nanofibres transforming thereby into crystalline nanowire-in-nanotubes. In addition, the magnetic properties of the Ni_0.5Zn_0.5Fe₂O₄ nanowire-in-nanotubes were also examined. It is believed that this nanowire-in-nanotube (sometimes called core–shell) structure, with its uniform size and well-controlled orientation of the long nanowire-in-nanotubes, is particularly attractive for use in the field of nano-fluidic devices and nano-energy harvesting devices.