Layered oxides, ruthenium-substituted Li2MnO3, were synthesized at 800°C and 1200°C. Their phase relation and electrical and electrochemical properties were investigated. Li2Mn1−x Ru x O3 synthesized at 800°C clearly separated into two phases, manganese-rich and ruthenium-rich phases, except for the narrow composition range of 0≤ x ≤0.05, while Li2Mn1−x Ru x O3 synthesized at 1200°C formed two solid solutions in the whole composition range across a structural transition between x =0.6 and 0.8. The electrical resistivity of Li2Mn1−x Ru x O3 decreased with increasing ruthenium content. Li2Mn0.2Ru0.8O3 (x =0.8) synthesized at 1200°C showed the lowest resistivity of 5.7×102 Ωcm at room temperature. The discharge capacity and cycling performance were improved by the ruthenium substitution. Li2Mn0.4Ru0.6O3 (x =0.6) exhibited a discharge capacity of 192mAhg−1 in the initial cycle and 169mAhg−1 in the tenth cycle with high and almost constant charge–discharge efficiencies of 99% from the second to tenth cycle at a current rate of 1/10C. The ruthenium substitution to Li2MnO3 is quite effective to improve electrical conductivity and charge–discharge performance.