The relationships between mechanical characteristics of rock and microcosmic mechanism at high temperatures were investigated by MTS815, as well as the stress-strain behavior of granite under the action of temperatures ranging from room tem-perature to 1200 °C. Based on a micropore structure analyzer and SEM, the changes in rock porosity and micro structural mor-phology of sample fractures and brittle-plastic characteristics under high temperatures were analyzed. The results are as follows: 1) Mechanical characteristics do not show obvious variations before 800 °C; strength decreases suddenly after 800 °C and bearing capacity is almost lost at 1200 °C. 2) Rock porosity increases with rising temperatures; the threshold temperature is about 800 °C; at this temperature its effect is basically uniform with strength decreasing rapidly. 3) The failure type of granite is a brittle tensile fracture at temperatures below 800 °C which transforms into plasticity at temperatures higher than 800 °C and crystal formation takes place at this time. Chemical reactions take place at 1200 °C. Failure of granite under high temperature is a common result of thermal stress as indicated by an increase in the thermal expansion coefficient, transformation to crystal formation of minerals and structural chemical reactions.
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