We report the application of thermal mechanical analysis (TMA) to study the crystallization behavior of poly(ether ether ketone) (PEEK). We are able to show that the two crystallization stages of PEEK are clearly distinguished by measuring the variation of film thickness with time during isothermal crystallization. These two crystallization stages can not be readily distinguished by DSC. The distinction by TMA becomes less clear with increasing crystallization temperature due to overlap of these two crystallization stages.