<p>Engineering plastics have received increasing attention because of their excellent mechanical strength, thermal stability, and chemical resistance. In this study, poly(ether ether ketone) (PEEK), a representative super engineering plastic, was investigated to evaluate the effects of the crystallization pathway and temperature on its aggregation structure. Although the crystalline structure of PEEK has been widely studied, limited discussion exists regarding how different crystallization pathways, specifically cold crystallization from the glassy state versus melt crystallization, affect structural development. Here, we examined the influence of both the crystallization pathway and the crystallization temperature on the degree of crystallinity and crystallographic lattice distortion. Under the experimental conditions employed in this study, regardless of the crystallization pathway, the degree of crystallinity increased with increasing crystallization temperature, whereas the degree of lattice distortion decreased monotonically. These results indicate that the aggregation structure of PEEK is governed by the crystallization temperature rather than by whether crystallization occurs from the glassy or molten state. This study highlights the critical role of thermal molecular motion during crystallization in determining the final structure. These insights provide a foundation for the more rational processing and design of high-performance thermoplastic components based on semicrystalline engineering plastics such as PEEK.</p>