High performance aramid fibers display their outstanding high tenacity, high modulus and high temperature resistant under various section. Kevlar129, PBO and Kermel fibers are compared for thermal properties. It can be found that the PBO fiber has the highest thermal degradation temperature by TGA in nitrogen atmosphere, and its initial degradation temperature is 556.5 degrees C. Accordingly, when,heated under 100, 200, 300, 400 degrees C for 1.5h, the tenacity and the extension to break of Kevlar129, PBO and Kermel fiber all decrease with temperature, but the modulus increase firstly then decrease. PBO generally has higher mechanical property retention. From the SEM observation to these fibers treated under heat, the morphology show their fracture failure mode when they are tensiled, and there exists fibrils in their core.