The work hardening behavior by cold rolling was investigated in ultralow carbon and low carbon martensitic steels containing 12%Cr or 18%Ni, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon steel, the hardness is almost constant irrespective of the reduction ratio. On the other hand, the low carbon steel exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.