The isothermal and continuous cooling transformation kinetics of bainite were studied using an automatic quench dilatometer. It was found that the overall isothermal bainite transformation kinetics were well expressed by Johnson-Mehl type equation and that the transformation rate decreased with increase in the austenite grain size. The analysis of transformation kinetics and the optical microscope observations suggested that the nucleation site of bainite was both on the grain boundaries and inside the austenite grain. This effect of austenite grain size on the rate of isothermal transformation to bainite was found to be smaller than that to pearlite. Comparison of the observed cooling transformation kinetics with that predicted from the isothermal kinetic data showed that the transformed fractions were additive in bainite transformation. This result might be understood if it were assumed that the bainite transformation proceeds with repeated nucleation and growth of basic subunits, each attaining the limit size rapidly.