In order to investigate the influence of water vapor pressure of surrounding environment on mode I fracture toughness of rock, a series of Semi-Circular Bend (SCB) tests under various water vapor pressures were conducted. The water vapor is the most effective agent which promotes stress corrosion of rock. The range of water vapor pressure used in this paper was from 10^-3 to 10³ Pa. The rocks used in this paper were African granodiorite (AG), Korean granite (KG), Kimachi sandstone (KS), Kumamoto andesite (KA) and Kunum basalt (KB). Measurement of elastic wave velocity and observation of thin section of these rocks were performed to make clear the micro structures, namely distribution of inherent micro cracks and grains. The results of the SCB test show that the fracture toughness of AG, KG, KA and KB were dependent on the water vapor pressure of the surrounding environment and decreases with increasing the pressure and that the fracture toughness of KS was independent on the pressure. It is considered that this decrease is due to stress corrosion promoted by water vapor. The experimental results also show that the degrees of influence of the water vapor pressure on the fracture toughness for AG was greater than the others. Together with the results and observed micro structures, it was concluded that the degree of the influence is dependent on the distribution and the density of inherent micro cracks.