In this article regularities of length changes of polyacrylonitrile (PAN) fiber, as well as its structure transformation into the structure of thermally stabilized fiber, have been studied in detail. The fiber has been manufactured with the use of dimethyl sulfoxide (PAN(1)) and dimethyl formamide (PAN(2)), with methylacrilate content of 5% (mass). Thermal stabilization has been carried out under isothermal conditions at 245, 255 and 265, 275, 285 and 290 degrees C in atmospheric air at constant load of 0.6 g/tex. By X-ray structural analysis it has been shown that the structure of PAN fiber crystal component is depicted by two kinds of coherent-scattering regions (CSR), differing in the average size by an order of magnitude. Phase transition of PAN into the new highly dispersed phase is accompanied by dispersion of PAN larger CSR. Qualitative and quantitative regularities of fine structure evolution have been determined at thermal treatment in the indicated temperature range. It has been shown that the transition of PAN(1) fiber into the new two-phase state during thermal stabilization progressed more actively than the same process in PAN(2), by similar to 25%. Increase of thermal treatment temperature leads to significant acceleration of PAN crystal phase dispersion and formation on the new highly dispersed phase. At that instance, average sizes of CSR of highly dispersed components of thermally stabilized fiber equal similar to 1 nm at the end of the process, when aromatization index of the material amounts to 51-58%.