In this experimental study, fracture toughness of heat-treated metal matrix composite was investigated. The gray cast iron was reinforced with steel wire of volume fraction of V(r) = 0.05 and three-point bend specimens were manufactured to determine fracture toughness. Heat treatment was applied to the specimens at the normalization temperatures of 850 degrees C and then cooled in three distinct environments (water, air, and furnace). Fracture toughness of the metal matrix composite was calculated by unloading compliance method. The study shows that the fracture toughness of the steel-wire-reinforced gray cast iron increases with the increase in cooling rate. Scanning electron microcopy (SEM) analyses were used to examine the microstructure and fracture surface. It is observed that the carbon diffuses from the gray cast iron to the steel wire and transition region having partially dissolved graphite was observed due to carbon diffusion, and it plays an important role in the fracture toughness depending on the cooling media.