In situ observations of the microscopic failure evolvement of sandstone under the coupled thermal-mechanical loadings were carried out by a scanning electron microscope (SEM). These results indicate that the effects of temperatures on cracking behavior and fracture toughness of sandstone are obvious. And, it is found that a critical transforming temperature is about 150°C for the fracture toughness of sandstone. That is, the fracture toughness value is to increase exponentially when the range of the temperature is from 25 to 150°C, but decreases exponentially when the range of the temperature is from 150 to 300°C. Therefore, in the present work the exponential relationships between fracture toughness and temperature are reported first. This is in good agreement with existing experimental evidence for the growth of micro cracks and thermal cracking.