It is proposed that an impregnation model can predict the porosity of composites for power-law fluids impregnation in fabric. The impregnation model takes the material properties and fabric characteristic dimensions into account. In order to verify the model, glass fabric reinforced polypropylene (PP) composites are fabricated by using compression molding procedure. The impregnation model is validated by a series of comprehensive experiments, and the experimental results are perfectly consistent with the impregnation model’s value. The impregnation process is studied respectively through the model and the experiment. The results of it have shown that the impregnation time can indeed be shortened for compression molding procedure by increasing the processing temperature or increasing the applied pressure.