Ultra-heavy cold-drawn pearlite wires provide an excellent combination of ductility and strength. Therefore, they have been widely used in engineering applications, such as suspension bridge cables, automotive tyre cords and cutting wires. In this work, the effects of low temperature annealing on the microstructure and mechanical properties of ultra-heavy cold-drawn pearlitic steel wires were investigated. The mechanical properties have been determined by tensile testing and the structures analyzed by TEM and HRTEM. The overall carbon contents in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by 3DAP. Experimental results show that, for the steel wires with strain (ε) less than 4, annealing in the range of 120~170 ℃ could effectively increase the strength of steel wires and remain most of the plastic performance. The tensile strength of wire with a strain of 3.0 can be increased about 150 MPa after annealing at 150 ℃ for 8 min. However, both of strength and toughness of steel wires with a strain 4.5 decreased after annealed at 170 ℃. After the steel wire is deformed by excessive strain (ε=4.5), the cementite decomposed obviously. DSC analysis showed that there is an obviously exothermic peak between 150 ℃ and 170 ℃ in the DSC curve. The TEM diffraction pattern analysis reveal the phenomenon of tailing at diffraction pattern, which is mainly caused by segregation of carbon atom at the dislocation after annealed at 150 ℃. However, HRTEM images show that annealing temperature as low as 170 ℃ could result in the transformation of partial cementite from amorphous state to nano-crystalline state. It could effectively pin and hinder the movement of dislocations. The underlying mechanism responsible for changes in microstructure and mechanical properties after annealing at low temperature are closely related to C-segregation and "crystal-amorphous" cementite transformation in heavy cold-drawn pearlitic steel wires.