NbB2-NbC composite powders were fabricated at reduced temperatures via the carbothermal route by mechanical milling of Nb2O5, B2O3 and C powder blends. Both powder blends containing stoichiometric amounts of the reactants and those milled were subsequently annealed in a tube furnace. The effects of milling time (0, 1, 3 and 5 h) and annealing temperature (1300 and 1400 degrees C) on both the formation and microstructure of the final products were examined. Increasing the milling time decreased the common formation temperature of the ceramic phases and contributed to the elimination of residual C from the final powders. The amount of the NbB2 phase increased and the amount of the NbC phase decreased in the composite powders by increasing the milling time and the annealing temperature. The predicted reactions and experimental results were compared, and the reaction mechanism of the Nb2O5-B2O3-C system was explained using thermochemical software, differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) and X-ray diffractometry (XRD). NbB2-based composite powders comprising NbC of approximately 600 nm in size were obtained with high purity from powder blends milled for 5 h and annealed at 1400 degrees C for 12 h. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.