The properties of the fiber material, which prepared from the high-performance heterocyclic polymer poly-p-phenylene-benzimidazole-terephthalamide (PBIA), were investigated. Poly-m-phenylene isophthalamide (PMIA) and poly-p-phenylene terephthalamide (PPTA) fibers were also studied, for comparison, under the same experimental conditions. Thermogravimetry(TG) and thermogravimetry coupled to Fourier transform infrared spectroscopy(TG-FTIR) were used to study the properties of these fibers. The results show that PBIA fiber has better tensile properties, thermal stability than that of PMIA and PPTA fibers. The onset degradation temperature of PBIA is the highest, namely 421°C in nitrogen. TG-FTIR provides information on the compositions of the pyrolyzates as well as their relationship to the structures of the polyamides. Analysis of the results indicates that a hydrolytic mechanism plays a leading role at lower temperature, and a homolytic mechanism is dominant at higher temperature.