Abstract: Electronic structures of Sm₅Co₂₈Si₆ cluster were studied by using spin-polarized MS-Xα method.The results show that a few of electrons are transferred to Sm（5d⁰） orbit because of orbital hybridization between Sm and Co atoms.The exchange interactions between 3d and 5d electrons lead to the magnetic coupling between Sm and Co,and that results in the long-range ferromagnetic order in SmCo_5.85Si_0.90.The calculated results for the cluster reveal that SmCo_5.85Si_0.90 is stable phase.Since the spin-up DOS peak of d electrons at E_F rises and the bonding of electrons at E_F strengthens with increasing Si concentration,the free energy of alloys decreases and the stable ferromagnetic order forms in SmCo_5.85Si_0.90.Considering the localization of 4f electrons and a few of 5d electrons,the magnetic moment of Sm will be 1.24μB which is in agreement with the experiments of Sm³⁺ ion-moment in metals（1.32~1.63μB）.