SAX (Satellite Astronomia Raggi-X) is the Italian Satellite devoted to study in the X-Ray wavelengths. X-rays in the range 300 keV to 0.1 keV will be observed over a wide field by means of 8 Scientific Instruments. The SAX orbit is circular with an altitude of 600 km and a maximum inclination of 5°. A life of 26 months is foreseen. The Thermal Control Design, which will make use of Passive Thermal Control elements ( Heaters, MLI, Paints), shall guarantee the best thermal environment for the Scientific Instruments. This includes not only control of the temperature level, but also of the magnitude of temperature gradients between different sides of the same Instrument. Naturally, the thermal control features must not obstruct the Instrument Field of View (FoV).The TCS shall also control the temperature of all the other Satellite equipment such as the Battery, Propulsion (Hydrazine) elements (Tank, Thrusters and Lines), and AOCS sensors. The Atomic Oxygen degradation typical in Low Earth Orbit (LEO) makes material selection more difficult. The TCS performance will be verified by thermal analysis using ESATAN TMM's and by thermal testing. A Development Test has already been carried out to evaluate the thermal performance of the most critical thermal design components. A complete Thermal Balance Test will be performed on a representative Thermal Model and the Flight Model will undergo a combined Thermal Balance/Thermal Vacuum Test. Each Test will be followed by a Correlation Analysis Campaign in order to validate the relevant TMM's and evaluate the adequacy of the adopted thermal design.