Abstract A new test and evaluation method has been developed to determine the critical fiber length characteristic of short fiber reinforced composites. From the so-called active ‘barbe’ (beard) length distribution calculated from the length distribution of the fibers in the composite and from the length distribution of the fibers protruding from the fracture surface, measurable characteristics have been derived, from which the critical fiber length can be determined. In order to demonstrate the applicability of the method, some measurements and calculations have been performed for injection molded glass fiber reinforced poly(ethylene-terephthalate) samples and the critical length determined has been compared with that obtained from droplet pull-out tests. It has been concluded that the new method provides an easily evaluated result, which is closer to reality than those obtained by other methods.