Short fiber reinforced thermoplastics are used increasingly as structural materials because they don’t only provide superior mechanical properties, but they can also be easily produced by rapid extrusion compounding and injection molding processes. Fiber breakage usually occurs in short fiber reinforced thermoplastics processing and resulting final products have Fiber Length Distribution (FLD). The fiber length is governed by a number of factors including fiber content and processing conditions. The mechanical properties of short fiber reinforced thermoplastics such as strength, elastic modulus and fracture toughness, are critically dependent on the fiber length distribution. In this project, parameters governing fiber length distribution, rheological properties and mechanical properties of the injection moulded specimens were investigated. This research work has been divided in three sections: 1) Fiber distribution length and average length were determined. 2) Rheological properties of the composite material for several length distribution and fiber content were measured. 3) Mechanical properties of injection moulded specimens were determined. The obtained results indicate that: - The investigated samples are visco-plastic fluids. The Herschel-Bulkley model that has three variables ? 0 , k n, is the best model for describing samples behavior. Although change in fiber length distribution doesn’t change the rheological model that define the fluid behavior, but it changes the parameters values of this model. Consistency index ,k, and yield stress ,? 0 , are directly related to fiber length distribution factor, while Non- Newtonian index, n, shows the inverse relationship with fiber length distribution factor. - In all specimens, stress and melt viscosity increase for higher fiber volume fraction. Furthermore, by decrease of fiber length distribution factor, viscosity and stress are significantly decreased. It should be noted that, in the composite rheological behavior, fiber length is a more effective parameter than fiber volume fraction. - The results of tensile and bending tests shows a direct relationship between fiber length distribution factor and composite strength and modulus. For mechanical properties of composite, fiber volume fraction is more effective parameter than fiber length distribution factor. On the other hand, bending modulus and strength are higher than their tensile values and have a greater range of changes with less influence by fiber length distribution factor. Key words: Short Fiber Composites, Fiber length distribution factor, Rheology.