Nanosized LiNi0.9Co0.1O2 powders used in lithium-ion batteries are successfully prepared via a water-in-oil microemulsion process. The average particle sizes of the microemulsion-derived LiNi0.9Co0.1O2 powders are in nanometer scale. The obtained powders are much smaller in size than the specimens prepared via the conventional solid state and sol-gel processes. Oxygen has significant enhancement effects on the cationic ordering of the calcined powders. Highly cation-ordered LiNi0.9Co0.1O2 powders with a layered R (3) over barm structure are obtained after heat-treatment at 800 degrees C in O-2. In addition, the high intensity ratio of I-003/I-104 reveals that lithium ions and transition metal ions are regularly situated at the 3a and 3b sites, respectively, rendering the high cationic ordering. The discharge capacity of the first cycle for the specimen calcined at 800 degrees C in O-2 is 170.9 mAh/g. After 20 cycles, the capacity retention of LiNi0.9Co0.1O2 powders is 93.2%, indicating that LiNi0.9Co0.1O2 powders with good cycling characteristics are obtained via the microemulsion process.