This paper reports a simple one-step hydrothermal routine to prepare orthorhombic LiMnO2 powder for Li ion battery application. Employing Mn2O3 and LiOH as the starting materials, hydrothermal reaction operated under 160°C for 12h generated pure-phased o-LiMnO2 powder. The morphological change and reduction in grain size between the reagent and the resultant were revealed by SEM observation, which indicated that LiOH played two important roles in the process, one as the Li ion source to form orthorhombic LiMnO2 by intercalation and the other as the corrosive medium to control the morphology and reduce the particle size. Detailed investigation showed that the LiOH concentration and the hydrothermal temperature were two key factors influencing phase purity of the final product. Pure-phased o-LiMnO2 prepared under optimized hydrothermal conditions showed higher capacity and better cyclical performance than the commonly prepared o-LiMnO2 powder, and therefore promised potential application for lithium ion secondary batteries.