Mixing of polyamide 6 (PA 6) and polyamide 66 (PA 66) is integrated in the trend of development of new and improved materials by combination of different polymers and some reinforcing materials to polymer composites. The specific polymer composite PA 6/A 66 reinforced with short glass-fibers combines the good coloring of PA 6, and the small moisture absorption of PA 66. Technical applications of PA 6/PA 66 composites are mainly used in the automotive industry. Specific requirement of this industry lead to the necessity to optimize the material resistance against crack propagation of the PA 6/PA 66 composites, using mechanical and fracture mechanical methods. So, the present investigations focus on fracture mechanics toughness optimization of the PA 6/PA 66 composites, including unstable and stable crack growth. The aim of this toughness optimization is to find out the optimal mixing ratio of PA 6/PA 66. Applications of PA 6/PA 66 in the automotive industry and Specific client wishes are the main reasons for black-coloring of the PA materials. The influence of several black-colorants (carbon black, nigrosine, spinel, iron oxide) on mechanical and fracture mechanical properties of the PA composites is also investigated using fracture mechanical methods. As experimental fracture mechanical method, preferentially, the instrumented Charpy impact test (ICIT) and the new Cut method to determine the stable crack growth of glass-fiber reinforced materials was used. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2245-2252, 2009