inetics of isothermal transformation of supersaturated phase of ordered a2 was investagated with method of quantitative metallographic examination and X- ray diffraction. The relationship between the amount of 7 phase precipitation and the isothermal transformation time with the both method is agreed. The rate of isothermal transformation in alloy 45Al is larger remarkably than the one in alloy 42Al. Under the eutectoid temperature in Ti- Al phase diagram the rate of transformation decreases with temperature,ho it is.not able that the isothermal transformation curve becomes the shape of letter C.