Hot stage transmission electron microscopy is applied to determine the growth mechanism during decomposition of austenite in hypo-eutectoid Fe-C austenites. The austenite-ferrite interface is mostly curved and moves sluggishly with periods of acceleration and deceleration. In some cases the interface is nearly straight and effectively immobile. Then, migration takes place by means of ledges which displace parallel to the immobile straight interface. The ledges migrate at a rate equal to the migration rate predicted for diffusion controlled migration. The highest migration rates observed for the curved interface are nearly equal to that calculated for diffusion controlled growth. The observed succession of periods of acceleration and deceleration for the curved interface is not predicted in the common theories for interface mobility during phase transformation. Detailed examination of region around the interface indicate that stress build up and stress relaxation are responsible for the deceleration and acceleration respectively. The stresses are due to the volume misfit between the ferrite formed and the parent austenite.