||Understanding the thermal impact on the hydromechanical behaviour of the host rock is one important issue in the design of underground storages for high level radioactive wastes. The increasing temperature inevitably affects the rock mass behavior with the generation of overpressure and associated mechanical effects. Consequently, it is essential for the designer of an underground storage to have appropriate knowledge of the coupled thermal and hydromechanical behaviour of the rock mass. The French Radioactive Waste Management Agency (ANDRA) has carried out some research on the mechanical and hydraulic phenomena of rock masses subjected to temperature. The main investigation was performed in the Meuse/Haute Marne Underground Research Laboratory (M/HM URL), built in a Callovo-Oxfordian claystone formation. An in situ thermal experiment called TER is performed in the M/HM URL and studied in this paper. The concept of disposal cells studied by ANDRA for high level activity wastes consists of horizontal micro-tunnels 40 m long and 70 cm in diameter, in which temperature could reach up to 90 °C at the wall. In TER experiment, the level of temperature is similar to the one forecast for concept. The experiment is monitored with several temperature, pore water pressure and deformation/displacement sensors. Prior to the field test, the Mohr–Coulomb constitutive model and the FLAC3D numerical code are used for the prediction and interpretation of the measurements. The influence of in situ stress state and a set of basic parameters involved in thermo-hydromechanical processes are discussed via a parametric study. The principal results of the in situ measurements and the numerical analyses of the thermo-hydromechanical behaviour of the underground storage are presented in this paper.