The effects of eight types of spheroidizing annealing conditions including annealing temperature, annealing time, cooling rate, and gas atmosphere in the annealing furnace on the microstructure were determined in SM45C steel which has been widely used for automotive parts. The well-developed spheroidized structure and minimum hardness were obtained when the steel was heat-treated 6 hours at $740^{circ}C$, cooled to $710^{circ}C$ at a cooling rate of $24^{circ}C/h$, and then kept for 7 hours at the $710^{circ}C$ followed by air cooling. In order to increase the productivity and to save the manufacturing cost, it is desirable to apply a faster cooling rate in the spheroidizing annealing. It was found that air cooling was the fastest cooling rate applicable to the SM45C steel. The steel heat treated in air showed the decarburized layer of about $110{mu}m$ in thickness at the surface of the specimen, resulting in serious problems in the quality of the quenched product.