The high-pressure crystallized poly (ether ether ketone) (PEEK) samples were investigated using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The results showed that the thermal stability of PEEK could be improved through appropriate high-pressure treatment. Although the 10wt.-% loss temperature and maximum decomposition temperature of the PEEK samples decreased with the increase of crystallization temperature and crystallization time, the effect of the applied pressure on preventing them from further degradation was obviously observed. Furthermore, the amorphous region of certain PEEK samples was eroded off after high-temperature degradation, which made the details of the spherulitic structures revealed more clearly using traditional etching technique. PEEK spherulites with different characteristics were disclosed with SEM, including those with sheaf-like structures, quench haloes or interpenetrating fibrils. The study presented here may be instructive to the applications of the polymer under such ultimate environmental conditions as high pressure and high temperature.