In order to investigate the influence of water vapor pressure of surrounding environment on 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 rocks used in this study were granodiorite and sandstone, which are known as anisotropic rocks. Measurement of elastic wave velocity and observation of thin section of these rocks were performed to make clear orientation of inherent micro cracks and grains. Then two types of specimens were prepared based on the results of the observed anisotropy. The experimental results show that the fracture toughness of the granodiorite is dependent on the water vapor pressure of the surrounding environment and that that of the sandstone is independent on the water vapor pressure. Furthermore the fracture toughness of both rocks has anisotropic properties. Then crack initiation was analyzed using images of fracture within fractured specimen taken by X-ray CT scanner. Together with the results of the experiments and the analyses of CT images, it was made clear that the anisotropy of fracture toughness is dependent on the micro structure of the rocks.