There is a deep interest in methods to fabricate hollow nanocrystals for potential application as high-efficiency catalysts or drug-delivery agents. Tubular one-dimensional nanocrystals have been prepared for a wide variety of materials, including semiconductors(1,2), metals(3,4), ferroelectrics(5,6) and magnetite(7). They can be produced by rolling up layered materials or via an axial growth in a rolled-up form(8-10), coating pores in templates(11) or by eliminating the core of a core-shell nanowire(1,7). The Kirkendall effect, a classical phenomenon in metallurgy(12), was recently applied to explain the formation of hollow spherical nanocrystals(13-17). Although the experimental demonstration and theoretical treatment mainly concern binary compounds and planar interfaces or nanoscale spherical interfaces, the fabrication route provided by the Kirkendall effect should be generic, and should also work for high-aspect-ratio hollow cylinders ( that is, nanotubes) or even more complex superstructures. In this letter, we report, for the first time, on ultra-long single-crystal ZnAl2O4 spinel nanotubes ( total diameter: similar to 40 nm, wall thickness: similar to 10 nm) fabricated through a spinel-forming interfacial solid-state reaction of core-shell ZnO-Al2O3 nanowires involving the Kirkendall effect. Our results simultaneously represent an extension of applying the Kirkendall effect in fabricating hollow nano-objects from zero-dimensional to multidimensional, and from binary to ternary systems.