A study of the kinetics of pearlite spheroidization under static annealing conditions was carried out in two materials — AISI 1080 steel and pure Fe-C alloy. A stereological “shape factor”,F, defined asF =Svp/3• Km, was introduced for the kinetic study. The significance of this shape factor in relation to the geometrical characters of lamellar structures is discussed. For constant temperature a linear relation betweenF and the logarithm of time was obtained. Analysis of the time and temperature dependencies for a constant shape factor gave an activation energy of 70 kcal/mole for AISI 1080 steel and 58 kcal/mole for Fe-C alloy which indicates that volume diffusion of Fe in ferrite is the rate-controlling mechanism. The modified fault migration theory, which was developed from the mechanism study of this research, was applied to predict the kinetics of the pearlite spheroidization. For both the AISI 1080 and the Fe-C alloy experimental results have a good match with the theoretical prediction.