A new perovskite related structure was found in the sample of nominal composition of Ba0.5Sr0.5CuOz which was synthesized at a high pressure of 6 GPa. The space group for the new structure was determined to be Pmmm by means of a transmission electron microscopic observation. The lattice parameters were: a=3.92 A ̊, b=3.94 A ̊ and c=7.93 A ̊. In this structure, a cation ordering was revealed by means of a structural simulation. The ordering sequence of atomic layers was as follows: [Sr-CuO2-Ba-CuO2-Sr]. Although no evidences for superconductivity were obtained in the present work, it should be emphasized that compounds of this structure would exhibit superconductivity because CuO2 planes were contained in them.