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• An unreported aramid fiber (AF)/PEEK composite is produced. • A simple surface modification method for the AF/PEEK composites is proposed. • A multiscale flexible-rigid CNT-COOH/ANF interphase has been constructed to improve interfacial adhesion. • The flexural strength (132.60 %) and ILSS (182.97 %) of AF/PEEK composites were enhanced. The application of aramid fiber (AF)/polyetheretherketone (PEEK) composites is currently hindered by the inert surface and poor wettability of AF, resulting in weak interfacial adhesion and poor mechanical properties. Surface coating and the introduction of nanostructures have been proven to be effective approaches to address this problem. Herein, a simple hybrid sizing agent has been developed to modify the AF surface, consisting of soluble polyimide (PI) as a compatibilizer, carboxyl-functionalized carbon nanotubes (CNT-COOH) as a rigid unit, and aramid nanofibers (ANF) as a flexible component. The synergetic effects of PI and the multiscale flexible-rigid structure (CNT-COOH/ANF) contribute to the formation of chemical and physical bonds between AF and PEEK matrix, further improving the interfacial adhesion and stress transfer efficiency. Attributed to the enhanced wettability and roughness of AF, compared with unsized AF, the flexural strength (220.97 MPa), modulus (13.26 GPa), ILSS (13.36 MPa), and storage modulus (12.93 GPa) of the AF/PEEK composite increase by 132.60 %, 99.00 %, 18.97 %, and 82.70 % respectively. Additionally, the flexible-rigid nanonetwork facilitates the penetration of the PEEK resin into pore spaces. This simple and effective approach exhibits promising potential in enhancing the interfacial bonding of AF/PEEK composites. [Display omitted] [ABSTRACT FROM AUTHOR]