Collagen-rich natural leather powders (PPd) with average particle size of 12 mu m were modified and dispersed in a polyurethane (PU) matrix. Three kinds of surface modification techniques were employed; surface polyaddition of PU, copolymerization with acrylamide (AAm), and grinding of the PPd with PU powder in a planetary ball mill. Surface modification was confirmed mainly by FTIR signals from the modifier. The DTA exothermic peak shifted toward a higher temperature by 260 K when copolymerized with AAm. On the copolymerized sample, the IR peak due to C=O stretching appeared between those of the original AAm and PPd, indicating the mutual chemical interaction. The tensile yield stress and critical strain of the composite films increased by surface modification with PU, while the latter decreased for the film containing the PPd powders copolymerized with AAm. Judging from the micrographs of the expanded films, the increase in the adhesive strength between the powder surface and the matrix after modification by AAm was larger than those by PU.