Sub-micro spinel LiNi0.5−x Mn1.5+x O4 (x <0.1) cathode materials powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UACP) method. The structure and electrochemical performance of this as-prepared powder were characterized by powder XRD, SEM, XPS, CV and the galvanostatic charge–discharge test in detail. XRD shows that there is a small Li y Ni1−y O impurity peak placed close to the (400) line of the spinel LiNi0.5−x Mn1.5+x O4, and the powders are well crystallized. XPS exhibits that the Mn oxidation state is between +3 and +4, and Ni oxidation state is +2 in LiNi0.5−x Mn1.5+x O4. SEM shows that the prepared powders (UACP) have the uniform and narrow size distribution which is less than 200nm. Galvanostatic charge–discharge test indicates that the initial discharge capacities for the LiNi0.5−x Mn1.5+x O4 (UACP) at C/3, 1C and 2C, are 130.2, 119.0 and 110.0mAhg−1, respectively. After 100 cycles, their capacity retentions are 99.8%, 88.2%, and 73.5%, respectively. LiNi0.5−x Mn1.5+x O4 (UACP) at C/3 discharge rate exhibits superior capacity retention upon cycling, and it also shows well high current discharge performance. CV curve implies that LiNi0.5−x Mn1.5+x O4 (x <0.1) spinel synthesized by ultrasonic-assisted co-precipitation method has both reversibility and cycle capability because of the ultrasonic-catalysis.