Inter-dimer magnetic interactions (J(ab(D))) between (BEDT-TTF)(2)(+) (.) dimers in the kappa-(BEDT-TTF)(2)Cu-2(CN)(3) crystal has been studied theoretically. In these crystals, triangular lattice structure with spin frustration has been discovered and experimental studies have been performed by several research groups. We have calculated the inter-dimer Jab(D) values by using several methods. In addition, we have analyzed the spin lattice with optimization of h angle in classical Heisenberg spin model and with numerical diagonalization in quantum Heisenberg spin model.