The composition of 4,4' Thiodiphenol (TDP) of Poly (cether ether ketone),(PEEK) copolymers can be measured by characteristic IR-absorption/peak of sulfurgroup. The DSC thermograms and x-ray diffraction patterns show the crystallinity of PEEK is drastically decreased with the incorporation of thio-groups in the backbones. However, it dose not alter the lattic structure of PEEK,which means the thio-containing part of PEEK blocks are totally/excluded from the crystalline structure and the crystals are throughly formed by PEEK, The introduction of the the thio-groups, although decreases the processing temperature, can also decrease the the thermal degradation temperature.