AbstractSummary: A series of molecular composites of PA 6/PA 66 was synthesized via in situ polymerization. The impact resistance of PA 6 was improved dramatically by incorporating a minor amount of PA 66 (2–10 wt.‐%), without decreasing the tensile strength. Inserting PA 66 macromolecules at a molecular level into a PA 6 matrix may interfere with the arrangement of the hydrogen bonds of PA 6, in turn changing the crystalline structure and impeding the crystallization of PA 6.SEM micrograph of the fractured surface of a PA 6/PA 66 composite containing 10 wt.‐% PA 66.imageSEM micrograph of the fractured surface of a PA 6/PA 66 composite containing 10 wt.‐% PA 66.