In the κ-(BETS) 2 FeCl 4 crystal as one of organic magnetic-metal crystals, localized spins of d-orbitals in FeCl4- anions is expected to play an important role to magnetic properties. To investigate all the magnetic interactions between neighboring two FeCl4- anions, various spatial conformation of dimers were assumed and studied. For variety of configurations of FeCl4- anion dimers, the values of effective exchange integral (J) in the Heisenberg spin model were calculated by using ab initio MO methods. It was found that very weak ferromagnetic interaction existed in a certain conformation, though antiferromagnetic interaction was reproduced in most of all conformations.