Terthiophene (3THP) is evaluated as an electrolyte additive for improving cyclic stability of lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode for high energy density lithium-ion batteries. Charge/discharge tests demonstrate that 3THP is effective for improving the cyclic stability of LiNi0.5Mn1.5O4. With applying only 0.25% 3THP in a standard (1M LiPF6 in EC and DMC, 1/2 in volume) electrolyte, the capacity retention of Li/LiNi0.5Mn1.5O4 cell after 350 cycles at 1C (1C=147mAg−1) under 4.9V was improved from 50% to 91%, which is among the best that have been reported in literatures although the content of 3THP is far lower than those achieved by applying other additives. The physical characterizations from scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction, indicate that a thin cathode film has been formed on LiNi0.5Mn1.5O4 particles, which suppresses the decomposition of electrolyte and protects LiNi0.5Mn1.5O4 from structural destruction.