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Three types of rock specimens, three-point bending specimen, anti-symmetric four-point bending specimen and direct shearing specimen, were used to achieve Mode Ⅰ , Mode Ⅱ and mixed mode Ⅰ - Ⅱ fracture, respectively . Microscopic characteristics of the three fracture modes of brittle rock were studied by SEM technique in order to analyze fracture behaviors and better understand fracture mechanisms of different fracture modes of brittle rock. Test results show that the microscopic characteristics of different fracture modes correspond to different fracture mechanisms. The surface of Mode Ⅰ fracture has a great number of sparse and steep slip-steps with few tearing ridges and shows strong brittleness. In the surface of Mode Ⅱ fracture there exist many tearing ridges and densely distributed parallel slip-steps and it is attributed to the action of shear stress.The co-action of tensile and shear stresses results in brittle cleavage planes mixed with streamline patts and tearing ridges in the surface of mixed mode Ⅰ - Ⅱ fracture. The measured Mode Ⅱ fracture toughness K Ⅱ c and mixed mode Ⅰ -Ⅱ fracture toughness Kmc are larger than Mode Ⅰ fracture toughness KⅠc. KⅡc is about 3.5times KI c, and KmC is about 1.2 times K Ⅰ c.