Formation of secondary ferrite in high carbon steel wires was investigated in order to suppress delamination in torsional deformation. The addition of 0.04mass% Nb to 0.98mass% carbon steel markedly increased the volume fraction of secondary ferrite, and delamination occurred in the steel filament that was drawn up to 3.8 in strain. However, the addition of 20ppm boron with 0.03% Ti to 0.99mass% carbon steel significantly decreased the secondary ferrite. Moreover, delamination did not occur in this steel filament which was heavily drawn up to 4.2 in strain. From microstructural and chemical analyses, free boron seemed to suppress the generation of the secondary ferrite, which can provide the initial crack site of delamination. The authors propose that the control of such initial crack site is a very important way to suppress delamination in high strength high carbon steel wires.