An advanced heat-resistant fiber (trade name Ekonol) spun from a nematic liquid crystalline melt of thermotropic wholly aromatic poly(p-oxybenzoate-p,p＇-biphenylene terephthalate) has been subjected to a dynamic thermogravimetry in nitrogen and air. The thermostability of the Ekonol fiber has been studied in detail. The thermal degradation kinetics have been analyzed using six calculating methods including five single heating rate methods and one multiple heating rate method. The multiple heating-rate method gives activation energy (E), order (n), frequency factor (Z) for the thermal degradation of 314 kJ mol(-1), 4.1, 7.02 x 10(20) min(-1) in nitrogen, and 290 kJ mol(-1), 3.0, 1.29 x 10(19) min(-1) in air, respectively. According to the five single heating rate methods, the average E, n, and Z values for the degradation were 178 kJ mol(-1), 2.1, and 1.25 x 10(10) min(-1) in nitrogen and 138 kJ mol(-1), 1.0, and 6.04 x 10(7) min(-1) in air, respectively. The three kinetic parameters are higher in nitrogen than in air from any of the calculating techniques used. The thermostability of the Ekonol fiber is substantially higher in nitrogen than in air, and the decomposition rate in air is higher because oxidation process is occurring and accelerates thermal degradation. The isothermal weight-loss results predicted based on the nonisothermal kinetic data are in good agreement with those observed experimentally in the literature.