Austempering heat treatment was applied to gray cast iron in order to study the effect of matrix toughening on fracture mechanics behavior. Copper alloying was applied to increase the hardenability of the metal. The as-cast material was austenitized at 900 degrees C for 1.5 h, and then austempered at 300 degrees C/3 h or 360 degrees C/2 h so as to obtain different matrix morphology, namely, lower ausferrite or upper ausferrite. A mixed ausferrite structure was also achieved by austempering at 360 degrees C for 8 min followed by 300 degrees C for 172 min. The results showed that the plane strain fracture toughness (K-iC) of the gray cast iron so heat-treated were increased from the as-cast of 12.3 MPa root m to that of 16.0, 23.8, and 26.1 MPa root m, respectively These were masked improvement of 30, 93, and 112% aver the as-cast material. Optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were applied to correlate the microstructural features to the properties attained.