The physical modification of the dry leather using supercritical carbon dioxide (SC-CO2) was studied in this article. A series of loosening processes of the leather fibers were carried out by changing the experimental conditions such as experimental pressure, experimental temperature, and time. The samples were characterized by scanning electronic microscopy (SEM), Brunauer-Emmett-Teller (BET) and X-ray diffraction (XRD). SEM images show that samples were loosened by SC-CO2 and the leather fibers in micron size arrange more orderly after treatment. The BET surface area of treated samples increase from 1.67 m(2)/g to 6.33 m(2)/g with the changing of conditions. The optimal treatment conditions were determined. Moreover, XRD patterns indicate that aggregation structure of collagen fibers in the sample was altered by SC-CO2, and it can be found that the loosening of leather mostly happened in amorphous regions of collagen fibers. Besides, the percent breaking elongation of the samples was examined by means of a tensile analyzing test, and it indicates that the elongation at break of all the treated samples in SC-CO2 can increase to 128.2% compared with 95.9% of the original ones. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 4015-4022, 2009