The electronic density of states, spin-splittings and atomic magnetic moments of SmCo7-x Cux are studied by means of the spin-polarized multi-scattering Xα method. The results show that a few of the electrons can transfer to the Sm 5d0 orbital due to orbital hybridization between Sm and Co. The exchange interactions between 3d and 5d electrons lead to the magnetic coupling between Sm and Co, and therefore result in the long-rangeferromagnetic order in SmCo7-x Cux. The Curie temperature of SmCo7-x Cux is generally lower than that of thecorresponding pure Co, which may be explained by the weaker average strength of coupling between Co latticesdue to some negative exchange couplings mainly from the 2e site. The calculated results for the Sm5Co30Cu4cluster may lead to a better understanding of why SmCo7-xCux is stable phase. Since the negative coupling of the 2e sites becomes small and the d bond at EF becomes stronger in contrast to SmCo7, which results in decrease of the free energy of the system, the stable ferromagnetic order forms in SmCo7-x Cux.