A Thermo-Triax apparatus has been developed to facilitate research on petrophysical properties of rock samples under simulated geothermal reservoir conditions. The apparatus consists of control systems for vertical stress and horizontal confining pressure, a pair of independent pore pressure controllers for applying different upstream, and downstream pore pressures at bottom and top of rock specimens, an external heater and a data logging system. Permeability of rocks is measured using steady state and transient flow methods. The thermal expansion of metallic parts in the triaxial cell and the error introduced into the readings of the extensometers at high temperatures are calibrated via experiments on an aluminum specimen with known coefficient of thermal expansion. The possibilities of studying the effect of stress and temperature on permeability and compressibilities of porous rocks with the Thermo-Triax apparatus are presented with first data. The change of pore volume during the non-isothermal process between adjacent temperature levels as well as along the measurement of permeability at leveled temperatures is interpreted and calibrated. The thermal expansion of mineral grains during heating is verified with the data of pore volume change and the magnitude of thermal expansion of mineral grains is estimated and compared with reported values. The permeability measurements along different heating paths can be used to verify the temperature dependency of stress-dependent rock properties.