Effect of ferrite grain size on dislocation strengthening was investigated in low carbon steels (0.006%C-0.15%C) with various grain sizes from 1 to 100 µm. In specimens with slight deformation, dislocation density increases in proportion to the inverse of ferrite grain size. In the dislocation density range below 2×10¹⁴/m², dislocation density increases linearly against deformation strain but it tends to level off due to the dynamic recovery of dislocations when dislocation density has exceeded it. On the other hand, tensile tests revealed that yield stress follows the Hall-Petch relation for as-annealed specimens but follows the Bailey-Hirsch relation for cold rolled specimens. This means that flow stress depends on only the dislocation density regardless of grain size. As a result, it was concluded that the introduction of dislocations has been promoted with decreasing ferrite grain size and this results in the increase of flow stress in the uniform deformation region.