The effect of chemical substitution on improving cycle performance of orthorhombic LiMnO2 (o-LiMnO2) was studied at the nominal compositions of LiMn0.95−x M x Cr0.05O2; M=Al, Ga, Yb and In, 0.01<x<0.07. Samples obtaining single phase were synthesized by hydrothermal technique. As the result of studies, LiMn0.92In0.03Cr0.05O2 was confirmed as the optimum composition. The crystal structure refinement showed that chemical substitution varied the average ionic radii of cation sites in o-LiMnO2. The ratio of average ionic radius at Mn site to that of Li site of the optimum compound was 0.87. This value was larger than that of no-substituted LiMnO2; 0.84. TEM observation revealed that stacking faults were observed in the LiMn0.92In0.03Cr0.05O2 grains. It is assumed that these two factors, controlling ionic radii and introducing stacking faults, are key parameters for improving cycle performance. However X-ray diffraction data and discharge curves suggested that phase transformation from orthorhombic to cubic spinel occurred during charge–discharge cycle.