A modified kinetics model of ferrite and pearlite transformations under applied stress is proposed based on the Johnson-Mehl-Avrami (J-M-A) equation and is shown as f=1−exp[−b(θ)t^n(θ)], in which b(θ)=b(0)(1+Aθ^B) and n(θ)=n(0). The parameters A and B in the suggested model were determined by regression of the data from experiments in a 0.38Cro steel and a eutectoid carbon steel. The calculation results of the two steels show that the suggested model can precisely predict the onset and the end of ferrite and pearlite transformations under applied stress. The effect of applied stress on ferrite or pearlite transformation is remarkable and the constants A and B in the suggested model vary with the different steels and different transformations.