An ultrahigh-carbon steel containing 1.3 wt. %C was thermomechanically processed to obtain spheroidized carbides embedded in a ferrite matrix. Austenitizing treatments at temperatures between 770 and 870degreesC were carried out. The grain size and the carbide diameter remain the same after austenitizing which is attributed to the short time kept at temperature and the fast cooling rates to room temperature used. Fracture toughness at room temperature was determined. Dependence with sample size was found: the smaller is sample size the higher is fracture toughness. This is attributed to the large contribution of plastic zone at the crack tip in small size samples. Fracture toughness does not vary significantly with austenitizing temperature since the distance between carbides and the ferrite grain size is kept constant.