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℃, the mixture of bainite and pearlite at 500℃, and to pearlite at 600℃, The tensile strength of patented wire exhibited the highest value when the structure was pearlite, while the bainitic structure showed the lowest.