First, it is demonstrated that the shape change accompanying bainite formation is accommodated by plastic deformation in austenite. Secondly, taking into account the corresponding plastic work, which increases due to carbon enrichment and transformation shear strain, as an entropy production mechanism, a new criterion, depending especially on initial chemical composition and grain size of austenite, is proposed for the stop of isothermal bainite before the expected equilibrium fraction expected from the phase diagram.