The stability region of the infinite-layer compound (Ca1-xSrx)CuO2 has been investigated in detail along the composition line CaCuO2-SrCuO2 in the SrO-CaO-CuO system. The samples were prepared at 956 degrees C in air at ambient pressure. Single phase of the infinite-layer structure compound is found to lie at the intersection of two different phase regions and its stability region is extremely narrow near x = 0.16. Temperature also has pronounced effect on the stability of the infinite layer structure. A trial of doping into the infinite layer compound (Ca1-xSrx)CuO2 was performed by partial substituting (Ca, Sr) with lanthanum.