We calculate the photon circular polarization at the capture np → dγ for polarized thermal neutrons. The calculations are performed both in the zero-range approximation and with the wave functions for the Reid soft core potential. The contribution to the circular polarization, which is of the same origin as the nonadditivity of the nucleon magnetic moments, has been taken into account in two ways. The first approach is based on the known value of the deuteron magnetic moment. In the second we have obtained relativistic corrections for the magnetic moment operator for the two-nucleon system, and using them calculate both the deuteron magnetic moment and the circular polarization in the neutron radiative capture independently.