Phase transition and microstructure evolution during carbothermal reduction-nitridation of TiO2 in an open system were investigated using XRD, TGA, SEM and laser particle analysis device. The results show that, the phase evolution sequences are: TiO2 (anatase) -> TiO2(rutile) -> TinO2n-1(n >= 4) -> Ti3O5 -> Ti(N,O) -> Ti(C,N,O) -> Ti(C,N). In the reaction process, the predominant reaction mechanism is TiO2/C solid-solid reaction in the beginning and subsequent the gas-solid reactions mainly between oxides and CO, C and CO2. The synthesizing powders gradually become finer in form of uniform spherical particles with the formation of cubic phase. (C) 2010 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.