A wide ductility trough covering from 700 to 1100 is observed in the curve of Reduction of Area (RA) vs. tempetature for 16MnCr5 pinion steel. At 750℃, corresponding to the minimum of RA, it is grain boundary sliding that controls its hot ductility rather than usual Deforming Induced Ferrite (DIF), which can only appear just below 750℃ and slightly improve hot ductility. The volume fraction of ferrite is dependent on the strain and strain rate.Firstly a critical strain must be necessary for formation of DIF then with strain rate increasing, the volume fraction of DIF decreases but RA is elevated. In the γ phase region, hot ductility is seriously deteriorated because of grain boundary sliding promoted by oxidcs and sulfides at the grain boundary and recovered because of dynamic recrystallization at higher temperature; when strain rate increases, ductility is improved as there is insufficient time for cracks to propagate along the γ grain boundary as well as dynamically precipitating, and ductility trough becomes narrower because the temperature for onset of dynamic recrystallization decreases. In addition, γ→α phase transformation introduced by temperature drop before the tensile test encourages precipitation of AlN and impairs ductility.