The low carbon bainitic steels are control-rolled, relaxed for different time after finish rolling, and then quenched in water. The mechanical properties show that the yield strength is influenced by the relaxation time after finish rolling. Meanwhile, relaxation time also affects the morphology of bainite, the bainitic packet size can be refined to the finest after relaxed for 20s, the lath-like bainitic ferrite is thin and short, the boundary of lath-like bainite and packet which composed by several aligned laths is very sharp, and the retained austenite is fine and linearly alignment. If no relaxing and directly quenched, the bainitic lath and packet size would be coarse. The relaxation processes after finish rolling not only refine the bainitic packet size, but also influence the morphology of the bainite, which would play an important role on the mechanical properties of low carbon bainitic steel.