Stress-strain behaviour in tension and in torsion was studied in ferrite-pearlite and ferrite-bainite microstructures of C-Mn steel. The fatigue tests were performed under reverse torsional loading on hour-glass shape specimens. The cyclic plasticity in torsion was found at about 37% of the monotonic yield stress in both ferrite-pearlite and ferrite-bainite microstructures. The ferrite phase in the direction of maximum shear stress was the preferable site for crack nucleation. The ferrite-bainite microstructure showed better fatigue properties than the ferrite-pearlite microstructure.