The phase stability and the decomposition behavior for the high Tc superconductor, YBa2Cu3Ox (x=6.92), at 20°-1200°C in O2, air, N2 and H2 were studied by means of the thermogravimetry (TG), the differential thermal gas analysis (DTGA), the differential thermal analysis (DTA) and the liquid N2 quenching. In the air, O2 and N2, the phase transition and the decomposition of YBa2Cu3Ox take place according to the following five steps. Orthorhombic-YBa2Cu3Ox→Tetragonal-YBa2Cu3Ox(Tetra)→Tetra, BaCuO2→Tetra, Y2BaCuO5, BaCuO2→Y2BaCuO5, BaCuO2, CuO The temperature and the oxygen content of specimens, when the phase transition and/or the decomposition take place, decrease along with the lowering oxygen partial pressure. Under hydrogen atmosphere, the decomposition takes four steps commencing with TG measurement. At the final step (around 900°C), YBa2Cu3Ox decomposes to Cu metal, BaO and Ba3Y4O9. At the second step (450°C), Y2O3, Ba(OH)2⋅H2O, and Cu were already found. At the third (600°C) and the forth (750°C) steps, a phase unknown was detected. The formation of Ba(OH)2⋅H2O can be regarded to occur due to the reaction of BaO and H2O. The decomposition process under hydrogen atmosphere is found to be very complicated.