FeNi3 alloy nanostructures were synthesized by hydrothermal methods in the surfactant/n-octane/n-hexanol/water quaternary reverse microemulsion systems. The size and shape of the products could be controlled by changing the type and dosage of the surfactant. Spherical particles with a diameter of ca 75 nmwere prepared when polyethylene glycol 4000 (PEG4000) was used as the surfactant. Sea-urchin-like particle was obtained when cetyltrimethylammonium bromide (CTAB) was used as the surfactant. The single sea-urchin-like particles were composed of many nanorods with diameters of ca 42 nm and lengths of 0.4-1.2 μm. The as-synthesized products were characterized by powder X-ray diffraction (XRD), Mossbauer spectroscopy, scanning electronic microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and multi-purpose magnetic variable field translation balance (MM VFTB). Both the spherical and sea-urchin-like FeNi3 samples exhibited typical ferromagnetic behavior at roomtemperature. Their saturation magnetization values (Ms) were 114.4 and 97.4 emu·g-1, respectively, while their coercivity values (Hc) were 94.0 and 329.0 Oe, respectively.