Abstract: It is noted that several abnormal phenomena associated with the increase of ground temperature have been observed before large earthquakes. In this paper, the relations between geothermal variations and the preparatory process of large earthquakes are investigated theoretically. It is assumed that there is an elliptical-shaped crack in an infinite elastic medium as a geothermal channel as well as an earthquake fault, the pressure and the temperature distributed in the crack as a linear funtion of x and y are taken into account. From the view point of linear elastic fracture mechanics, the stress field and stress-intensity factor around the crack tip have been obtained in terms of the method suggested by Kassir et al.The result shows that thermal stress might be of the order of kilobar. The relations between the magnitude of thermal stress and the thermal expansion coefficient of the medium are analysed. Finally, the effects of variations of on the seismo-genetic process are discussed briefly.