The hydro-mechanical behavior of clay rock at high temperatures is quite different from that at ambient temperature. This chapter experimentally investigates the thermo-hydromechanical (THM) behavior of Boom Clay under heated conditions. Consolidated undrained triaxial compression and permeability tests were conducted under varying stresses and temperatures to assess short-term mechanical and hydraulic responses. Additionally, NMR tests were performed to analyze thermally induced microstructural changes. The results collectively clarify the mechanisms through which elevated temperature influences the hydro-mechanical properties of clay rock, providing essential data for the safety assessment of high-level waste disposal systems.