This study aimed to develop a mathematical model to predict the soil temperature at various depths and times of the year under the Egyptian conditions. This model was validated against two sets of measured soil temperature profiles. The validation results showed acceptable agreement. Experimentation of this model showed that soil temperature after a certain depth remains almost constant with time of the year and the depth of 4 m under the ground level implies 1.5 °C amplitude of soil temperature, while a further deepness (5 m) caused a slight improvement of 0.9 °C amplitude. Thus, a depth of 4 m could be the preferable depth to installation an earth tube for geothermal system.