The effect of the carbon content on the Hall-Petch parameter has been investigated for fully pearlitic steels with the carbon range of 0.52-0.92 wt%. The increase of the carbon content in pearlitic steels enhances tensile strength and hardening rate of cold drawn pearlitic steels by the refinement of the initial interlamellar spacing and the increase of the Hall-Petch parameter. The Hall-Petch parameter is not influenced by the initial interlamellar spacing of pearlite although refining the initial interlamellar spacing increases tensile strength and hardening rate during wire drawing. However, the carbon content in cold drawn pearlitic steels significantly affects the magnitude of the Hall-Petch parameter. The magnitude of the Hall-Petch parameter, k, is expressed as a function of the volume fraction of cementite (V-c, i.e. the carbon content); k = constant .V-c(1/2).(1-V-c), which shows a good agreement between experimental and calculated values.