The in-situ continuous cooling ␤-to-␣ transformation kinetics of extra-pure (EP) Ti and of grade-4 commercially pure (CP) Ti were investigated using a fully computer-controlled resistivity-temperature realtime measurement apparatus and transmission electron microscopy. The ␤-to-␣Ј martensitic transformation occurs under near pure shear condition, and the habit plane of lath-type martensite was determined to be parallel to , which is in good agreement with the prediction of the crystallographic theory. The M s temperature of EP-Ti was measured as 800 °C and can be raised by up to about 40 °C due to the generation of thermal stress and local deformation during rapid cooling. The massive transformation was, for the first time, observed to occur over a wide range of cooling rates in an EP-Ti. The massive start temperature and its occurrence were, unlike the martensitic transformation, hardly affected by the generation of thermal stress and local deformation during rapid cooling. The stable regime of massive transformation in a grade-4 CP-Ti was considerably shifted toward a slower cooling rate side and was significantly contracted at the same time. This is because the presence of iron impurity not only largely suppresses the massive transformation but also significantly delays a long-range diffusional transformation. 54336b(4150)a¿