A series of samples with nominal compositions La1-xBaxCu1O3-y was prepared by solid-state reaction. The subsolidus phase relation in the CuO-rich region of the La-Ba-Cu-O system has been determined by an x-ray diffraction method. Two ternary compounds were confirmed to exist in the system. The tetragonal or pseudotetragonal phase (Ba2-zLaz)La1Cu3O7+δ has a wide homogenous phase range from z=0.095 to z=0.578, and a crystal structure closely related to the structure of Ba2YCu3O7-δ. Superconductivity of compounds in this system was studied by resistivity and ac susceptibility measurements. The relation between crystal structure and the superconductivity is discussed.