The structural,magnetic and electronic properties of the double perovskite Ba2SmNbO6 (for the simple cubic structure where no octahedral tilting exists anymore) are studied using the density functional theory within the generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction.The total energy,the spin magnetic moment,the band structure and the density of states are calculated.The optimization of the lattice constants is 8.5173 (A),which is in good agreement with the experimental value 8.5180(A).The calculations reveal that Ba2SmNbO6 has a stable ferromagnetic ground state and the spin magnetic moment per molecule is 5.00μB/f.u.which comes mostly from the Sm3+ ion only.By analysis of the band structure,the compound exhibits the direct band gap material and half-metallic ferromagnetic nature with 100％ spin-up polarization,which implies potential applications of this new lanthanide compound in magneto-electronic and spintronic devices.