Abstract: Measurement of the mosaic structure for neutron monochromator using γ diffraction requires that the incident γ-ray beam has the features of big intensity, good energy uniformity and small angular divergence, so it is an essential procedure to design the shape and size of gold material of γ source. The cuboid shape of gold was chosen by analysis of the relationships between the intensity, divergence of the γ beam and the beam profile shape. The cuboid gold sizes in three directions were optimized as 0.6, 6 and 10 mm, respectively by analyzing the effect of γ-ray attenuation in gold on the beam intensity, and the effect of γ scattering in gold on the energy uniformity. Using the γ source of the cuboid gold material designed, the Ge crystal mosaic of neutron monochromator was measured successfully by the method of γ diffraction.