Ultrafine LiMn2O4/carbon nanotube (CNT) nanocomposite is synthesized by a one-step hydrothermal treatment. In the nanocomposite, LiMn2O4 nanoparticles of 10-20 nm in diameters are well crystallized and uniformly distributed in the CNT matrix. The CNTs not only provide a conductive matrix, facilitating fast electron transport, but also effectively reduce agglomeration of LiMn2O4 nanoparticles. The nano-LiMn2O4/CNT nanocomposite exhibits superior rate capability and cycling stability compared with the sol-gel synthesized LiMn2O4, making it promising for high-power applications.