A wide range of springs come in various shapes and sizes for use in a variety of equipment such as mobile phones and automotives. Compact and lightweight components are increasingly demanded as expectations grow for the reduction of environmental impacts by such equipment. As for the development of springs, one of those components, high-strength steel wires are required. For engine applications such as valve springs, wires also need to have appropriate heat resistance. We have successfully developed steel wire satisfying the requirements above by adding excess Si up to 1.0% on the conventional hard drawn steel wire (SWP-B, Japan Industrial Standard) (1). The developed Si-enriched wire demonstrates improvements in tensile strength by 10%, fatigue limit by 20% and heat resistance by 50% in comparison to the original wire. Our previous report (2) described the improvements, which are achieved by i) strengthening the solid solution of the Si in lamellar ferrite, and ii) reducing strains by high-temperature annealing. In this report, we discuss the reactions of cementite in pearlite lamellas.