Internal structures in a polypropylene reinforced with glass and carbon fibers were investigated by using the non-destructive X-ray CT (for computerized tomography), imaging technique. The mechanical properties of PP composite, are generally influenced upon the fiber length distribution (FLD), the aspect ratio(AR) and fiber orientation distribution (FOD), and etc. Three dimensional images were generated in which individual fibers were clearly resolved, along the injecting direction and the mold geometry. The quantitative FLD, AR and FOD data will be presented in this study.