Natural graphite, which is used as an anode material in lithium-ion batteries, is thermally treated to improve its cycleability and reduce irreversible reactions with the electrolyte. Natural graphite is treated in air at 550°C. The weight loss increases when the thermal oxidation time is increased. The BET surface area of the graphite decreases with increasing weight loss. The cycleability and efficiency of the thermally oxidized natural graphite improves significantly. Thermal oxidation decreases the irreversible capacity for side-reactions with the electrolyte on the first cycle. By contrast, it does not change the reversible capacity and rate capability. The improvement in the cycleability after thermal oxidation may be due to the removal of imperfect sites on the graphite.