The preparation of LiyMnxN1-xO2 from LiOH . H2O, Ni(OH)(2) and gamma-MnOOH under oxygen and flowing air is studied. Single phases of LiyMnxNi1-xO2 (0 < x < 0.3 in O-2, 0 < x < 0.2 in air) are prepared by using gamma-MnOOH as a manganese source. The optimum heating temperatures are 800 degrees C for x = 0.1 and 850 degrees C for x = 0.2. The particle size of the source manganese compound is an important factor fur the preparation of the single phase LiyMnxN1-xO2. It is found that LiyMnxN1-xO2 is formed from LiyNiO2 and Li2MnO3 at a temperature higher than 750 degrees C. As the average valence of transition metals (Mn and Ni) for LiyMnxNi1-xO2 obtained under flowing oxygen is found to be more than 3, it is experimentally confirmed that manganese in LiyMnxNi1-xO2 exists as both Mn(III) and Mn(IV), For this reason, the presence of LiyMnxNi1-xO2 would be allowed for a wide range of y, for example, 0.99 < y < 1.10 for LiyMn0.1Ni0.9O2.