Isothermal transformation behavior during patenting and variations of microstructure and tensile strength of patented wires were investigated in Si-added high carbon steel. The TTT curves of the steels were made for two different austenitizing temperature. As the salt bath temperature was increased, the observed microstructures were bainite at $450^{circ}C$, the mixture of bainite and pearlite at $500^{circ}C$, and to pearlite at $600^{circ}C$, The tensile strength of patented wire exhibited the highest value when the structure was pearlite. while the bainitic structure showed the lowest.