The diffuse scattering of beta-Ag3SI was investigated by neutron powder diffraction over the sin theta/lambda range of 0.1 to 0.7 Angstrom(-1) at room temperature. An oscillatory diffuse scattering has been clearly observed. The theoretical treatment including the effects of the correlation between the thermal displacements of atoms was applied to the analysis of the observed diffuse scattering. A strong correlation between the thermal displacements of nearest-neighbour iodine and silver atoms is necessary to explain the oscillatory diffuse scattering.