The electrochemical properties of the spinel LiNi0.5Mn1.5O4 cathode material are influenced by the synthesis processes, which determines the impurity phase and the distribution of Ni and Mn in the spinel structure. Taking advantage of neutron's high sensitivity to Ni and Mn, in situ neutron diffraction has been employed to quantify the phase formation/structural evolution process under continuous heating/cooling and isothermal annealing conditions. The results show that the subtle Ni and Mn ordering process occurs slowly at 700 °C and the degree of ordering can be controlled by the annealing time. At temperatures above 750 °C, the LiNi0.5Mn1.5O4 spinel phase starts to decompose into the rock-salt impurity phase accompanied by the release of O2. The rock-salt phase reverts back to the spinel phase upon cooling along with the oxygen uptake. The dynamic process of structural evolution of LiNi0.5Mn1.5O4 that was unraveled by in situ neutron diffraction is valuable for guiding the synthesis of cathode materials with desirable properties.