Abstract: TiB2-TiC composite powders were prepared at low temperature in the Ti-B system with PTFE (polytetrafluoroethylene) as a chemical activator. The effects of PTFE addition on reaction process and reaction mechanism were investigated. The results show that there was only unreacted titanium in the product when the addition of PTFE was less than 5wt.%; when the addition of PTFE was 5wt.%, TiC phase could be observed in the product, but the energy released from the reaction between titanium and PTFE was not enough to induce the reaction between titanium and boron; when adding 10wt.% PTFE was added, TiB2-TiC composite powders, which were less than 0.4 μm in average particle size and had high purity, could be synthesized via low-temperature combustion synthesis at 550 ℃. The reaction mechanism could be described as following: titanium reacts with PTFE at first, resulting in large amounts of heat released; subsequently, the released heat induces the combustion reaction between titanium and boron particles to form TiB2.