Abstarct: In this thesis, we have studied the structure, surface morphology, optical and magnetic properties of FeNi3 powder samples prepared in an easy and low cost hydrothermal method. For characterization of the samples, a field emission scanning electron microscope (FESEM) a X-Ray diffractometer , a vibrating sample magnetometer (VSM) and an UV-Vis spectrophotometer were used. Powder samples were prepared using iron chloride (FeCl2.4H2O), nickel chloride (NiCl2), sodium hydroxide (NaOH) and hydrazine hydrate (N2H4.H2O). The results from XRD patterns indicated formation of FeNi3 cubic crystal with Pm3m space group structure and lattice constant a = b = c = 3.537 Å. FESEM images showed the nanoparticles have spherical shape. Examination of the magnetic properties of the samples by VSM confirmed that the samples have a ferro-magnetic phase at room temperature. The band gap of the prepared samples was measured and the results showed the samples have bandgaps in the range of about 1.7 -2.7, which indicates the effect of NiFe2O4 secondary phase in samples.