This study was performed to find the composition area of cubic spinel-type monophase oxides composed of the Mn-Co-Ni ternary system. Starting materials were prepared by mixing Mn, Co, and Ni nitrates then evaporating to dryness. Each starting oxide was fired at 700, 800, 900, 1000, and 1100 degrees C in air. The regions of cubic spinel monophase (CSM) were confirmed to spread with decreasing firing temperatures. The region of CSM at 1000 degrees C was seen near the line connecting the points of Mn : Co : Ni = 2 : 4 : 0 and 4.5 : 0 : 1.5. The area at 800 degrees C spread toward Co and Ni, as compared to the results at 1000 degrees C. In the region containing more Mn above the area of CSM at 800 degrees C, the phase had tetragonal spinel or alpha-Mn2O3 besides cubic spinel structure. Below this area, the phase contained rock-salt-type crystal besides cubic spinel structure. This tendency at 1000 degrees C was the same as that at 800 degrees C.