LiMnO2 and LiMn0.9Cr0.1O2 were prepared by a high temperature solid-state route. In comparison with orthorhombic LiMnO2, monoclinic LiMnO2 was formed with Cr substitution. The research shows that the irreversible transformation to spinel-like structure during LiMnO2 cycling is a gradual phase transformation process. In comparison with the 2.9 V and 4 V plateaus of LiMnO2, LiMn0.9Cr0.1O2 shows only 2.9 V plateau for further cycling. So it is clear that the electrochemistry behavior change from LiMnO2 to LiMn0.9Cr0.1O2 phase is due to doping of Cr.