Lithium nickel manganese oxide of LiNi1/2Mn1/2O2, which is a one-to-one mixture of LiNiO2 and LiMnO2 in its chemical composition, has been prepared by an improved processing method. Cycle tests of a lithium cell with LiNi1/2Mn1/2O2 operated in voltages of 2.5–4.5V shows about 200mAhg−1 of rechargeable capacity without noticeable loss during 30 cycles. The thermal behavior of fully charged LiNi1/2Mn1/2O2 is also examined by differential scanning calorimetry (DSC) and shows that the exothermic reaction of □0.89Li0.11Ni1/2Mn1/2O2 (fully charged state under this experimental condition) is milder than that of □NiO2 or □Mn2O4. The results on rate-capability tests of a Li/LiNi1/2Mn1/2O2 cell show that 85% of discharge capacity observed at low rate discharge can be stored and delivered at 1h rate at present.