Equal amounts of Fe- and Ni-substituted Li2MnO3 (chemical formula: Li1+x [(Fe1/2Ni1/2) y Mn1−y ]1−x O2, 0< x <1/3, 0.2≤ y ≤0.8) were synthesized using coprecipitation–hydrothermal–calcination. Although the samples with y less than 0.5 are only monoclinic Li2MnO3-type structure (C2/m), samples with y larger than 0.6 show a two-phase nature consisting of the monoclinic phase and cubic LiFeO2 phase ( F m 3 ¯ m ). Electrochemical characterization as a positive electrode shows that the Li2O extraction region disappears above y =0.6 on initial charging and that the energy density is decreased drastically above the composition on initial discharging. The optimized transition metal ratios are y =0.4 and 0.5 because the initial average discharge voltage increases with y and the maximum initial cycle efficiency is attained. In the optimized composition, the Fe- and Ni-substituted Li2MnO3 is a 3.5V class positive electrode, with similar charge and discharge profiles to those of the most attractive active material, NMC positive electrode (chemical formula: Li1+x [(Co1/2Ni1/2) y Mn1−y ]1−x O2, 0< x <1/3, 0.2≤ y ≤0.8). Consequently, Fe can be used as an activator in combination with Ni for constructing “Co-free” Li2MnO3-based positive electrodes. The calcination-condition-dependence of electrochemical properties at the optimized composition is also examined. The effects of the Fe valence state on initial charge–discharge curves are discussed.