The surface of graphite powder with different grain sizes was modified with lanthanum ultraphosphate (LaP5O14) crystals by means of chemical process, and its oxidation resistance was investigated. LaP5O14 crystals were formed on the graphite powder by the present chemical process, and LaP5O14 crystals partially existed on the graphite powder. As the surface of the graphite powder was modified with LaP5O14 crystals, it showed 40-110 degrees C higher oxidation starting temperature (T-s) and oxidation completion temperature (T-c) than as-received graphite powder, and they did not depend on the concentration of LaP5O14 above 2.4 wt%. In an oxidation test at 900 degrees C in air, the oxidation ratio of graphite powder was decreased by the surface-modification with only 2.4 wt% LaP5O14. It is evidenced that the blocking effect of active sites on the graphite surface is probably dominant rather than the surface coating effect for the mechanism of the oxidation protection of the surface-modified graphite powder with LaP5O14 crystals. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B. V. and The Society of Powder Technology Japan. All rights reserved.