Abstract: In order to investigate the influence of the angle between the stress direction and the structural plane on the mechanical properties and fracture modes of carbonaceous slate, uniaxial compression, triaxial compression, Brazilian tensile strength, and uniaxial shear experiments were conducted on carbonaceous slate samples collected from the Muzhailing Tunnel of the Weiwu Expressway. The relationships between the angle and various mechanical and deformation parameters, such as compressive strength, tensile strength, shear strength, elastic modulus, and strain, were examined, and the variation patterns of mechanical and deformation characteristics are analyzed. The main results are as follows: (1) As the angle increases from 0° to 90°, the peak strength and strain of carbonaceous slate increase gradually, while the elastic modulus decreases gradually. (2) Under compressive loading, carbonaceous slate shows brittle failure, with four types of fracture modes identified: tensile fracture along the structural plane, shear fracture along the structural plane, tensile fracture through the structural plane, and Y-shaped tensile-shear composite fracture. (3) The angle between the stress direction and the structural plane controls the fracture mode and mechanical properties of carbonaceous slate, which is also the primary cause of anisotropy.