JFE Steel has promoted to utilize many thermodynamic software and databases in the last decades in view of the surmount importance of cost effective optimization of manufacturing processes as well as analyses and predictions of experimental results. For this, Technical supporter has been appointed for undertaking versatile calculations requested from various R&D sections in the company, and the number of requests exceeds well over 100 every year. This paper describes studies on phase transformation kinetics of austenite to pearlite phase in 0.4 mass%C-X(X=Cr,Ni). The isothermal transformation curves were analyzed with use of the Avrami-type Equation: X(t)=1-exp(-Rt^n) where X(t) is the fraction transformed and n and R are constants. The parameters R, so-called transformation rate constant of pearlite , were independent of the supercooling from A1 temperature and the driving force of pearlite transformation. These values were dependent on the energy differences of ferrite phase from supercooled austenite phase regardless of alloy compositions and transformation temperatures. This energy difference was equivalent to the driving force of eutectoid ferrite. The pearlite transformation rate of 0.4 mass%C steels was found to be controlled by eutectoid ferrite growth.