The changes in the fibrillar and the lamellar structure in nylon 6 fibers resulting from drawing and annealing were studied by a detailed analysis of their two-dimensional small-angle scattering patterns. The scattering object that gives to rise the diffuse equatorial scattering in the angular range of Q = 0.02 to 0.3 Angstrom(-1) is assumed to be a fibril. There are two distinct regimes in the equatorial diffuse scattering. The scattering at Q < 0.1 Angstrom(-1) is dominated by scattering due to the longitudinal dimension of the fibril, and that at Q > 0.1 Angstrom(-1) to the lateral dimensions/organization of the fibril. The interfibrillar regions, unlike the interlamellar regions that are essentially made of amorphous chain segments, may have microvoids in addition to amorphous chain segments. The intensity distribution within the lamellar reflections was used to obtain the lamellar spacings and the dimension of the lamellar stacks. The length of the fibrils is between 1000 and 3000 Angstrom, the higher values being more prevalent at lower draw ratios. The fibril length is larger than the length of the lamellar stack, and approaches the latter at higher draw ratios. Annealing does not change the lengths of the fibrils, but the length of the lamellar stack increases. The fibrils form crystalline aggregates with a coherence length of similar to 200 Angstrom at higher draw ratios. The diameter of the fibrils (50-100 Angstrom) determined from the lamellar reflection using both the Scherrer equation and the Guinier law are consistent with the lateral size of the crystallites derived from wide-angle x-ray diffraction. The longitudinal correlation of the lamellae between the neighboring fibrils improves upon drawing and decreases upon annealing. The degree of fibrillar and lamellar orientation is about the same as the crystalline orientation. Lamellar spacing increases upon drawing (from similar to 60 to 95 Angstrom) and annealing (from similar to 85 to 100 Angstrom). This is accompanied by an increase in the width of the amorphous domains from 30 to 50 Angstrom in drawn fibers, and from 45 to 55 Angstrom in annealed fibers. The diameter of the fibrils decreases slightly upon drawing and increases considerably upon annealing.