A series of novel PA 1212/PA 6 composites were synthesized via in-situ polymerization. Incorporating minor PA 1212 (2-12 wt %) into PA 6 matrix led to about 5 times increase of impact resistance of PA6 with its tensile strength maintenance. Inserting PA 1212 macromolecules into PA 6 matrix by this method may interfere with the arranging order of hydrogen bonding of PA 6 by changing its crystalline structure and impeding its crystallization. Different from the homogeneous molecular PA 66/PA 6 composites, PA 1212/PA 6 composites had a microfiber structure with width 100-300 nm and length several micrometers. PA 1212/PA 6 composites had better crystalline thermal stability and comprehensive mechanical properties than the corresponding molecular PA 66/PA 6. After tensile deformation, the lamellar bundles in PA 1212/PA 6 had a prior orientation along the tensile direction, while the orientation direction in pure PA 6 had an angle about 45 degrees with the tensile direction. The crystalline microstructure and morphology of PA 1212/PA 6, especially the microfibers, might play an important role in the toughening of PA 6 with maintenance of strength.