The influence of heavy drawing deformation (true strain similar to4.0) on the microstructures of pearlitic steel wires containing 0.9mass% of carbon has been investigated. The microstructures have been examined by transmission electron microscopy, atom probe field microscopy, X-ray diffraction and differential scanning calorimetry. In the lightly deformed (true strain similar to1.5) wires, typical dislocation structures are observed in the ferrite region and the cementite has poly-crystallite structures. Concentration of carbon in the ferrite region is less than 0.05 mass% and the cementite contains stoichiometric amount of carbon. Drastic structural changes occur, such as decomposition of cementite, decrease in dislocation densities and increase in local strains in the ferrite region, when the true strain exceeds 2.5similar to3. The degree of the cementite decomposition is strongly affected by the strain and about a half of the cementite is considered to be decomposed when true strain reaches 4. The amount of carbon emitted from those decomposed cementite corresponds to 0.4similar to0.5 mass% C, however, the concentration of carbon in ferrite region is only about 0.1 mass%. Therefore, it is likely that the excess carbon atoms originating from the decomposed cementite might be segregated at the interfaces between non-decomposed cementite and ferrite.