This article deals with the influence of the polymeric matrix, such as isotactic polypropylene (iPP), polyethylene (PE-HD), and isotactic polybutene-1 (iPB-1), and the glass fiber content on the material behavior of short glass fiber reinforced thermoplastics. The glass fiber content of all materials ranged between 0 and 50 wt %, which corresponds to a volume content between 0 and approx. 0.264. To describe the mechanical properties of all materials, the stiffness, strength, hardness, and toughness behavior were determined. The crack toughness behavior regarding unstable crack propagation was also assessed by applying fracture mechanics concepts. It was found that the energy-determined J-values for the PP material system reach their maximum at a glass fiber content of 0.135. In contrast, the crack toughness of the PE-HD materials increases continuously with increasing glass fiber content due to the unchanged deformation ability at simultaneously increasing strength. The toughness level of the PB-1. materials is nearly the same independent of the glass fiber content due to the opposite trend of the load and the deformation ability. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2093-2102, 2010