The description of nanostructures in polyacrylonitrile fibers (PANFs), stabilized fibers (SFs) and carbon fibers (CFs) is still ambiguous and even controversial. In this paper, small angle X-ray scattering (SAXS) technique was used to characterize the nanostructures in PANFs, SFs, and CFs. Grubb and Thunemann-Ruland methods were used to analyze the orientation of the anisotropic scatterers in the fibers, and Shioya-Takaku method was used to analyze the lateral dimension of the anisotropic scatterers. The 2D-SAXS patterns were well decomposed into a diamond-like component and a fan-shaped component based on the simulation of SAXS patterns. The results demonstrate that there are always some microfibrils that contribute to the diamond-like SAXS intensity distribution in the three fibers. Besides, there are some extra vacancies that contribute to the scattering background in the SFs. However, microvoids and microfibrils jointly contribute the SAXS patterns of CFs. The dimensions and orientation distribution of these scatterers have been obtained, presenting a structural hereditability from PANFs, to SFs, and then to CFs. In-situ SAXS studies on the three fibers were also performed under the axial tensile deformation. It has been found that the lateral sizes of microfibrils are almost unchanged, implying that the deformation breakage of the fibers is accompanied by the relative slippage of microfibrils. New structural models for the three fibers are given in this paper.