For the calculation of the spin-orbit (SO) contribution to zero-field splitting (ZFS) tensor, we derive the ZFS formula based on the two-component relativistic second-order Moller-Plesset perturbation theory with the second-order Douglas-Kroll-Hess (DKH2) Hamiltonian from the ZFS formula described by sum-over-states form. The present ZFS formula is obtained also from the ZFS formula (with a prefactor) based on the coupled-perturbed spin-orbit coupling (CP-SOC) method of Neese. For several organic magnetic molecules, the calculated ZFS parameters (D-SO and E-SO) by the present ZFS formula almost agree with those by the CASSCF/MRMP2 method. (c) 2012 Elsevier B.V. All rights reserved.