In this study, we have prepared nanopowders of Yttrium Iron Garnet (YIG). Phase formation and crystalline structure were investigated by X-ray diffraction (XRD) and IR spectroscopy. Room temperature saturation magnetizations (M_s) of the samples were evaluated using a vibrating samples magnetometer (VSM). The results show that the size of nanoparticles renge from 70 to 250 nm. The results of structure investigation show correlation between Lattice constant and particle size of samples. The Critical size was obtained about 200 nm.