The electronic density of states, spin-splittings and atomic magnetic moments of SmCo 5.85 Si 0.90 alloy have been studied using the spinpolarized MS-Xa method. The results show that a few of electrons are transferred to Sm(5d 0 ) orbital due to orbital hybridization between Sm and Co atoms in the alloy. The exchange interactions between 3d and 5d electrons are more important than that of the polarization effects of the conductive electrons, thus it is the main reason resulting in the long-range ferromagnetic order in SmCo 5.85 Si 0.90 alloy. There are some negative exchange couplings mainly occurring at Co(2e) site, which weaken the average strength of coupling between Co lattices. Thus, the Curie temperature of SmCo 5.85 Si 0.90 alloy decreases distinctly compared with pure Co. The calculated results for the Sm 5 Co 28 Si 6 cluster may lead to a better understanding of why SmCo 5.85 Si 0.90 alloy 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, which result in the free energy of the alloy decrease, the stable ferromagnetic order forms inside SmCo 5.85 Si 0.90 alloy. Considering the localization of 4f electrons and a few of 5d electrons arising from the orbital hybridization, the magnetic moment of Sm atom will be 1.24m B in SmCo 5.85 Si 0.90 alloy that is in agreement well with the experiments of Sm 3+ ion-moment and Sm atom-moment in metals.