Nanocubes and hollow cubic microcages of FeCo alloy have been synthesized in large scale by reduction of aqueous Fe2+ and Co2+ with hydrazine in the presence of poly(ethylene glycol) and cyclohexane at a relative low temperature, which were confirmed by X-ray powder diffraction, transmission electron microscopy, and scanning electron microscopy. The morphology and dimensions of FeCo alloy nanocrystals could be controlled by the reaction conditions such as the concentrations of cyclohexane and PEG-400, reaction time, and the molar ratio of Fe2+ to Co2+ in the solution. The concentration of iron in these particles could be varied between 17% and 56%. The highest magnetization of 211.9 emu/g was recorded for 68 ± 6 nm Fe50Co50 nanocubes. The work demonstrates that shape and size-controlled synthesis can offer a simple solution to fabricate magnetic FeCo nanocubes that are promising for single nanoparticle recording and for high performance exchange-spring nanocomposite magnetic applications.