Composite materials consisting of reduced graphene oxide and LiNi0.5Mn1.5O4 were in situ prepared by a simple one-step hydrothermal treating method. The physical property and electrochemical performance of the composite materials were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, charge/discharge testing, and electrochemical impedance spectroscopy. The results demonstrate that the graphene oxide is partially reduced and uniformly in situ anchored on the surface of LiNi0.5Mn1.5O4. As a result, the specific surface area of the composite material dramatically increases from 0.2488 to 8.71 m2 g−1, and the initial specific discharge capacity improves from 125.8 to 140.2 mAh g−1, respectively. Furthermore, the capacity retention maintains 95.8% after 100 cycles, and the electrode polarization has significantly been lessened. At rates of 1, 2, and 5 C, the composite material with 5% reduced graphene oxide can deliver much higher capacities than the pristine LiNi0.5Mn1.5O4. Moreover, AC impedance test results show that the interfacial charge transfer impedance obviously reduced. It is confirmed that the introduction of reduced graphene oxide through hydrothermal treating is effective to enhance the electrochemical performance of the composite material.