In order to find out the effect of cementite decomposition on delamination, decomposition behavior of cementite in high carbon steel wires during drawing and aging was investigated by atom probe field ion microscope, transmission electron microscope and torsion testing. Fracture mode of normal fracture in torsion test is Mode III and that of delamination is Mode II. Microvoids or coarse grain boundary ferrite are not observed in the delaminated wires. Cementite decomposition proceeds with the drawing strain and aging, and the carbon concentration of lamellae ferrite remarkably increases, which can only be due to the dissolution of cementite and concurrent migration of carbon atoms into ferrite. The measured carbon concentration varies from region to region, depending on the lamellae spacing or the location within wires. It is considered that cementite decomposition proceeds heterogeneously in macroscopic and microscopic scale. Delamination occurs in the regions in which the maximum carbon concentration of ferrite exceeds 1 at%, irrespective of the dry drawn, the wet drawn and the aged wires. The heterogeneous decomposition of cementite suggests that the strength of ferrite is non-uniform. It may be concluded that delamination is caused by the non-uniformity of ferrite strength. Namely, delamination results from the local shear stress concentration in the low ferrite strength regions, which correspond to the locally low carbon concentration areas.