Abstract The supercritical organic Rankine cycle (SORC) system is a promising solution for the recovery of low-grade heat resources and for the utilization of geothermal energy. The thermal stability of the organic working fluid is one of most important criteria for establishing the SORC. Therefore, it is very important to understand the mechanism of thermal decomposition of the organic working fluid. In this study, decomposition experiments and theoretical calculations using the density functional theory (DFT) method were performed to evaluate the thermal stability and elucidate the mechanism of decomposition of a new-generation working fluid, (1E)-1,3,3,3-tetrafluoropropene (HFO-1234ze(E), CFH CHCF3). The main decomposition products included pentafluoroethane (CF2HCF3), 1,1,1-trifluoroethane (CH3CF3), and 2,3,3,3-tetrafluoropropene (CH2 CFCF3). HF, the key molecule, is produced and consumed multiple times throughout the course of the decomposition reaction. HFO-1234ze(E) decomposed to C1 C2 hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) under moderate operating conditions. On the other hand, under harsher operating conditions, the formation of long-chain HFCs and HFOs with C3 C5 was experimentally observed, where these species can be produced from the reactions between the decomposition intermediates.