The mechanical properties of fiber reinforced thermoplastics depend on the characteristics of the fibers, such as their three-dimensional orientation distribution, which is the result of the processing conditions and of the initial products before mixing. Different composites were obtained by injection of reinforced isotactic polypropylene containing 30 wt% fibers. Different size distributions of fibers were investigated. Their average lengths range from 60 to 470 mu m. Specific new tools were developed by combining microscopy and image analysis to determine the microtextural characteristics of the fibers, such as their size distribution or their orientation in the matrix. Our technique allows the following of the size distribution evolution of the fibers as the composite is processed. The three-dimensional orientation of the fiber is established using the in-plane angle and the angle of inclination. The evolution of the three-dimensional orientation distribution is studied in the sample, by differentiating central and border behavior and also as a function of fiber size. It is concluded that the coupling of scanning electronic microscopy with image analysis is an interesting technique that allows the 3-D orientation of the fibers to be determined. This technique is validated by showing that the results obtained agree with those that other authors obtained on comparable materials.