Bainitic transformation occurs in a temperature range between the high temperature transformations such as ferrite and pearlite and the low temperature transformations such as martensite. Bainite consists of aggregates of platelets or laths of ferrite separated by regions of phases such as austenite or martensite. The aim of this research is determination of the kinetics of the austenite transformation over the temperature range 250–500 • C. For this purpose, several specimens from a plain-high carbon steel were isothermally heat treated at different temperatures from 250 to 500 • C for different holding times. The specimens then were studied using a transmission electron microscope. The results showed that austenite, in the high carbon steel, transformed to bainite over the temperature range 250–475 • C, according to separate C-curves on the isothermal transformation diagram. It was shown that above ∼325 • C, austenite transformation starts with formation of upper bainite and below that temperature the transformation starts with formation of lower bainite. Also, in some regions in the temperature range 250–375 • C both upper and lower bainite could be seen, simultaneously.