Mechanical alloying (MA) of Ti-25.0~37.5at%Si powders was carried out in a high-energy ball mill, and in situ thermal analysis was also made during MA. In order to classify the synthesis behavior of the powders with respect to at%Si, the synthesis behavior during MA was investigated by in situ thermal analysis and X-ray diffraction (XRD). In situ thermal analysis curves and XRD patterns of Ti-25.0~26.1at%Si powders showed that there were no peaks during MA, indicating Ti 5 Si 3 was synthesised by a slow reaction of solid state diffusion. Those of Ti-27.1~37.5at%Si powders, however, showed that there were exothermic peaks during MA, indicating T i5 Si 3 and Ti 3 Si phase formation by a rapid exothermic reaction of self-propagating high-temperature synthesis (SHS). For Ti-27.1~37.5at%Si powders, the critical milling times for SHS decreased from 38.1 to 18.5 min and the temperature rise, ΔT (= peak temperature - onset temperature) increased form 19.5 ? C to 26.7 ? C as at%Si increased. The critical composition of Si for SHS reaction was found to be 27.1at% and the critical value of the negative heat of formation of Ti-27.1at%Si to be -1.32 kJ/g.