The high temperature organic Rankine cycle (ORC) has recently attracted much interest for its excellent performance in renewable energy utilization and industrial waste heat recovery. The thermal stability of working fluids is the important property for fluid selection studies because of the possible decomposition at high temperatures. Siloxanes are good selections for high temperature ORCs in previous studies. However, study on the thermal stability of siloxanes for high temperature ORCs is scant. This paper studied the thermal stability of siloxanes, using hexamethyldisiloxane (MM) as a representative fluid. An experimental method with two test systems was designed to identify the compositions of decomposition products. Linear siloxanes, such as octamethyltrisiloxane (MDM), were found to be the main decomposition products of MM. The influences of pressure, temperature, and time on the decomposition were then determined experimentally. The decomposition mechanism of MM and the influences of the decomposition on ORCs were also analyzed based on the experimental results.