The dimensional variations of PEEK with temperature are quantified by the vertical displacement of a load placed on the sample. We introduced a very simple concept which corresponds practically to the scheme that is given by Wendlandt in 1974. In the thermal behavior of the PEEK, the influence of the load applied to the sample as well as the method of preparation of the samples and their pre-treatment are examined. The loads used correspond to the following stress: 0.57 x 10(3), 2.5 x 10(3), 4.30 x 10(3) and 5.70 x 10(3) N/m(2). Cylindrical samples are prepared from the powder by compression and by thermal treatment up to the temperature of fusion of the substance. Some pellets prepared by compression are treated up to the fusion temperature before studying their deformation. The results show that under the temperature of fusion of the material, the deformation is weak and not influenced by the mechanical stress used. However, above this tempertaure, a threefold expansion of PEEK is observed between 350 and 530 degrees C and the deformation becomes influenced by the mechanical stress applied. Otherwise, the deformation of the sample obtained by compression is smaller than the deformation of the sample prepared by heat treatment. Finally, the dimensional variations of the sample pre-treated up to the fusion differ from those of the un-treated sample.