In the present theoretical study we investigate the structural, magnetic and bonding properties of Ru12 and Ru13 clusters. Our results show that the Ru12 cluster is non-magnetic and is more stable than the Ru13 cluster. The bonding analysis reveals that the stability of Ru12 can be attributed to the multiple covalent bonds formed from the d and s-d hybrid orbitals of Ru atoms resulting in three Ru4 squares stacked one above the other forming a cuboid. This stacking is stabilized by the p bonds due to the side overlap of d orbitals. The non-magnetic behavior of Ru12 is related to the pairing of electrons due to the strong covalent bonding.