<p>Commonly, plain carbon fresh martensitic steel shows high strength and breaking immediately. However, low C – 2wt.% Si – 5wt.% Mn fresh martensitic steel exhibits excellent strength and ductility. But it’s not clear why 5 wt.%Mn martensitic steel has ductility more than plain carbon martensitic steel. In this study, the effect of C and Mn on strength and ductility is investigated by in-situ X-ray diffraction during tensile tests in SPring-8. The relationship between the work hardening behavior and the dislocation behavior is analyzed. The dislocation density was calculated with modified Williamson-Hall method and modified Warren-Averbach method. XAFS measurement was also performed to investigate the interaction between Mn and C. It was found that the increase rate of dislocation density in plain carbon martensitic steel was higher than in 5%Mn martensitic steel. Adding 5%Mn, the increase rate of dislocation density can be reduced. We found that the tensile strength is determined by the upper limit of dislocation density, and the uniform ductility is determined by the increase rate of the dislocation density. The dislocation arrangement parameter obtained by XRD and TEM observation revealed that adding 5%Mn inhibited the formation of dislocation cell structure, resulting in increasing dislocation density. From XAFS results, it is considered that the attractive distance between Mn atoms and C atoms increases the interparticle distance of C atoms fixed to dislocations. Therefore, it is considered that the shear stress on dislocation for breaking through between the solute atoms decreases, and the work hardening rate decreases.</p>