Cubic cobalt-substituted magnetite CoxFe3-xO4 nanocubes (NCs) with uniform composition distributions of Co, Fe and O in the NCs, obtained via solution synthesis, are reported in this paper. Through the control of the reaction conditions, the size of the cubic NCs could be tuned from 35 to 110 nm. It was found that the cubic shape could easily induce the (400) orientation of the NCs on a Si substrate, and applying an external magnetic field in the out-of-plane direction could further enhance the (400) orientation of these NCs on the Si substrate. The highest coercivity of 2.07 kOe could be obtained by assembling the NCs in the external magnetic field. The reported magnetic cobalt-substituted magnetite NCs provide an ideal class of building blocks for studying ferrimagnetic nanoparticle (NP) assemblies with easily controlled magnetic alignment for magnetic tape recording with ever increased areal storage density.