Abstract: The mode I fracture toughness is a main mechanical parameter of rocks, and it is influenced by many factors. The rock's mesoscopic bedding angle is one of the factors, which is not well studied in China. The three-point bending beam (CB) method is used for the metamorphic sandy slate rock obtained in the tunnel excavation, for the mode I rock fracture toughness and the compressional wave velocities of rocks, lateral, longitudinal and without mesoscopic bedding, respectively. It is shown that there is a close correlation between the K_\rm IC , the fracture damage deflection, the mesoscopic bedding, and the compressional wave velocity. Meanwhile, the concept of anisotropic coefficient Y based on the test results has been proposed, the K'_\rm IC calculated by the anisotropic coefficient Y is in good agreement with the K_\rm IC measured in the actual test.