Nanosized orthorhombic LiMnO2 (o-LiMnO2) with high purity and good crystallinity has been synthesized by a simple process under mild conditions (80℃) involving controlled oxidation of Mn (OH) 2 followed by in situ ion exchange of lithium ions with the resulting intermediate MnOOH (in situ oxidation-ion exchange). By studying the effects of varying the lithiating agent, the oxidant and washing conditions, a mechanism for the synthesis of nano o-LiMnO2 by this method has been proposed. Nano o-LiMnO2 obtained by in situ oxidation–ion exchange has a high capacity with an initial discharge capacity of 220 mAh g-1 and a good cycling performance with a reversible capacity of 194 mAh g-1 after 30 cyclings, indicating that it is a potential synthetic method for the positive electrode material of lithium ion secondary batteries.