Mechanical alloying (MA) of Ti-50.0~66.7at%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-50.0~59.6at%Si powders showed that there were exothermic peaks during MA, indicating TiSi, TiS 2 , and Ti 5 Si 4 phase formation by a rapid exothermic reaction of self-propagating high-temperature synthesis (SHS). Those of Ti-59.8~66.7 at%Si powders, however, showed that there were no peaks during MA, indicating any Ti silicide was not synthesised until MA 240 min. For Ti-50.0~59.6at%Si powders, the critical milling times for SHS increased from 34.5 min to 89.5 min and the temperature rise, Δ T (=peak temperature-onset temperature) decreased form 26.2 ? C to 17.1 ? C as at%Si increased. The critical composition of Si for SHS reaction was found to be 59.6at% and the critical value of the negative heat of formation of Ti-59.6at%Si to be -1.48 kJ/g.