Crystallization process of Li1+xMn2-xO4 from mixture of Li2CO3 and Mn2O3 was studied by high temperature XRD which is suitable to observe high temperature phases directly. Thermal behavior of the lithium manganese oxides was investigated in the temperature range between 298 and 973 K. In the case of mixture of stoichiometric composition (Li/Mn = 1/2), lattice parameter of the cubic spinel decreased linearly during cooling process without appearance of any secondary phases. On the other hand, for lithium rich composition (Li/Mn = 1.1/1.9), the lattice parameter deviated from the straight relation at higher temperature, and this might be related to reversible phase separation into Li2MnO3 and Li1+yMn2-yO4 (0 < y < 0.1).