The solid solution of nickel oxide and manganese oxide has been synthesized successfully by heating Ni1−a Mn a OOH at a temperature ranging from 350 to 1000°C in oxygen atmosphere, and investigated as a high-capacity negative active material for lithium secondary cells. The discharge capacity of nickel and manganese oxide solid solution was decreased with increasing heat-treatment temperature. The average discharge potential shifts toward negative as increasing Mn content in the solid solution. Meanwhile, large amount of Mn in the oxide solid solution caused a poor cycleability. The Ni0.75Mn0.25O1.36 obtained by heating its raw material at 600°C delivered a large discharge capacity over 700mAhg−1 with a relatively low average discharge potential of 1.69 versus Li/Li+. In addition, the Ni0.75Mn0.25O1.36 gave the best capacity retention of 91% with representative charge–discharge curves even after 22 cycles. According to the results of XRD and high-resolution X-ray fluorescence spectrometer (HRXRF) measurements for the oxide solid solution before and after the first charge, it was clear that an amorphous-like or nano-sized phase was formed during the first electrochemical reduction; all of the nickel and a part of manganese were reduced to metallic state after charge to 0.2V versus Li/Li+.