GGG polycrystalline material was prepared by co-precipitation method with the metal Ga and Gd2O3 as the starting materials and aqueous ammonia as the preprecipitator. The X-ray diffraction spectra of GGG polycrystalline material prepared by co-precipitation method ,Nd:GGG prepared by solid-state method and the crystals GGG and Nd: GGG grown by Czochralski method were measured. Their lattice parameters were computed by extrapolation method. The lattice parameter of GGG polycrystalline material is smaller than that of Nd:GGG polycrystalline material, it is probably because the Gd3+ substitute Ga3+ and Nd3+substitute Gd3+ partly. Meanwhile, The lattice parameter of Nd:GGG crystal is larger than that of GGG crystal, from the result we know that Nd3+ substitute Gd3+. In addition, the lattice parameters of crystals are bigger than those of polycrystalline materials, which is probably caused by Gd3+ substitute Ga3+ due to Ga component vaporization. These results show that Ga component vaporization maybe exists not only in the preparation of polycrystalline materials but also in the crystal growth, so the Ga component vaporization should be considered in polycrystalline material preparation and crystal growth,The vaporization of Ga component can be controlled by using co-precipitation method