Pore size distributions of acid collagen fibers were calculated by the micropore method and the Dollimore-Heal method from the adsorption isotherm of nitrogen in order to elucidate the change in pore volume through acidifying the alkali-processed collagen fiber, The micropores with radii of 12 Angstrom or larger appeared after acidifying the raw collagen fiber C-O with pore radii larger than 20 Angstrom. Increases in pore volume were greater within radii of 12-15.5 or 15.5-24.5 Angstrom rather than within radii of 24.5-95 Angstrom. It was suggested that the interspaces of the triple helix or microfibril within the raw collagen fiber or alkali-processed collagen fibers were increased due to the formation of carboxylic acid moieties during acid preparation.