Nano-sized composite powder which consisted of two manganese-based oxides, alpha manganese dioxide (α-MnO2) and spinel Li–Mn–O, was successfully formed by intergrowth of the spinel phase inside α-MnO2. This composite oxide was synthesized by precipitation and heat treatment in air; α-manganese dioxide powder was firstly prepared by oxidative precipitation of Mn(II) with K2S2O8 in an aqueous solution, and then a mixture of the obtained manganese oxide powder and LiOH methanol solution was heat-treated in air. Electron microscopy and diffraction observations confirmed that the manganese oxide composite consisted of nano-sized grains of the spinel LiMn2O4 and α-MnO2 phases. It was found that this α-MnO2/spinel LiMn2O4 composite electrode exhibited highly reversible lithium insertion compared to the pristine α-MnO2 and conventional LiMn2O4, that is, the composite demonstrated high discharge capacity of 148mAhg−1 as a cathode material of lithium cells in the potential range of 2.5–4.3V with no significant capacity fading. It was thought that the intimately mixing of two oxides on a nanometer scale helped to maintain structural integrity on charge–discharge cycling, which leads to excellent capacity retention for both of the spinel and alpha-type manganese oxide.