We investigate the structural stability and electronic properties of ordered perovskite-type compounds Ba 2MIrO 6 (M = La, Y) by use of density functional theory. Cubic ( Fm-3 m), rhombohedral ( R-3) and monoclinic ( P2 1/ n) phases are considered for each compound. It was found that the most energetically stable phase for Ba 2YIrO 6 and Ba 2LaIrO 6 is P2 1/ n and R-3, respectively. It is also interesting to find that Ba 2YIrO 6 in R-3 phase, which was not reported in experiment, has a slightly lower energy than experimentally observed cubic Fm-3 m phase. The calculated results also show that Ba 2YIrO 6 in Fm-3 m, R-3 and P2 1/ n phases and Ba 2LaIrO 6 in Fm-3 m phase are half metallic, while Ba 2LaIrO 6 in R-3 and P2 1/ n phases show semiconducting behavior. The calculated spin magnetic moments indicate that the pentavalent iridium ion has a low spin configuration ( t 2 g 4 ) both in Ba 2YIrO 6 and Ba 2LaIrO 6.