There are small cracks in rocks; therefore, when rocks undergo loading, stresses are concentrated at the tip of cracks causing rock fracture before reaching its ultimate strength. The critical value of the stress intensity factor at the crack tip is called fracture toughness. The tensile strength of rocks is weak; therefore, Mode I (tensile mode) is the most critical loading mode. In some cases, rocks continuously experience heating-cooling. Therefore, it is necessary to determine the effect of heating-cooling cycle number on mode I fracture toughness, which is the objective of this research. To achieve this objective, we conducted three-point bending test on semi-circular specimens of three types of natural rock including sandstone, limestone and andesite to determine the mode I fracture toughness. A series of concrete specimen was also tested for further investigation. The specimens were heated up to 700° C in 1, 5 and 10 cycles and then cooled. A series of experiments were also conducted on the specimens at room temperature (25 ° C). According to the rising of temperature in firing process, the rate of temperature rise for specimens in the electric furnace is determined to be 15 ° C per minute. Fracture toughness of andesite rock, sandstone and limestone specimens decreases under cyclic conditions. Results indicate the generation and expansion of micro fractures in some rocks after undergoing cycles of heating-cooling, which causing an increase in the effective porosity and decrease in the P-wave velocity in rocks.