For the X-ray structure analyses of polymers, a concept of the crystallite structure factor was defined, in which the structure factors are not only calculated on a unit cell but also summarized over the whole of a crystal. According, it has been found that X-ray diffraction intensities are varied with the shape and size of small crystallites as well as with the atomic coordinates. This method gave the new structural information on the crystalline region of synthetic fibers such as nylon and polyester.