High capacity, stable Fe3O4/Fe nanocomposites for Li-ion battery anodes were manufactured via heat-treating Fe3O4–C (amorphous) nanoparticles that were made via a continuous hydrothermal flow synthesis (CHFS) reactor. Compared to analogous Fe3O4 nanoparticles, the Fe3O4/Fe nanocomposite anodes (vs. Li/Li+), displayed a high specific capacity of ca. 390 mAh g−1 after 50 cycles, at a modest current rate of 200 mA g−1 (at the highest Fe metal content). The performance of the Fe3O4/Fe materials at higher current rates was also excellent (ca. 260 mAh g−1 at the highest current rate of 2000 mA g−1), which confirms that the presence of Fe metallic particles can significantly improve cycling stability of Li-ion battery anodes by retaining structural metal oxide integrity.