Phase equilibrium in the La-Sr-Cu-O system has been studied mainly by the X-ray diffraction analysis. The equilibrium phase diagram of the La2O3-SrO-CuO system was drawn up at 1173K in air. We found two new compounds; Sr3Cu5Ox and La4SrCu5Ox. Sr3Cu5Ox has an orthorhombic structure with lattice parameters; a=3.956, 6=13.412 and c=11.475 in A. U. It was confirmed that the K2NiF4-type oxide (La1-αSrα)2CuOx is a solid solution of 0≤α≤0.5 in which the terminal compounds are La2CuO4 and LaSrCuO4, respectively. In contrast with an orthorhombic La2CuO4 (α=0), (La1-αSrα)2CuOx of 0.1≤α≤0.5 has a tetragonal structure. (La1-αSrα)2CuOx coexists with CuO, La4SrCu5Ox and La2SrCu2O6 for 0≤α≤0.05, 0.05≤α≤0.13 and 0.13≤α≤0.5, respectively. The dependence of lattice parameters of the solid solution (La1-αSrα)2CuOx on α was also determined.