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Background:. Skin contracture after skin grafting is undesirable. It is generally accepted that full-thickness skin grafts contract less than split-thickness skin grafts. However, unexpected secondary skin-graft contracture sometimes occurs after full-thickness skin grafting. We tried to elucidate the causes of skin contracture from the viewpoint of the orientation of collagen fibers to find a way to reduce skin-graft contracture. Method:. First, we examined the collagen fiber orientation of the skin over the whole body in Sprague-Dawley rats. Next, two pieces of skin (width: 30 mm × 30 mm; thickness: ca. 2 mm) were stripped off a rat for grafting. The pieces were grafted to different sites so that the collagen fibers of the graft and surrounding skin ran parallel or perpendicular to each other. The collagen fiber orientation before and after the skin grafting was determined using Osaki’s microwave method, a mechanical method, and scanning electron microscopy. Results:. The rat skin exhibited marked variations in collagen fiber orientation among different sites. The direction of the collagen fiber orientation corresponded to that of minimal mechanical strain. We found that the collagen fiber orientation in skin grafts remained almost unchanged after skin grafting. Conclusions:. Mismatched collagen fiber orientation between grafts and the surrounding skin is considered to be a cause of secondary contracture after skin grafting. We propose that skin grafts that minimize the difference in collagen fiber orientation between the skin graft and the surrounding skin should be selected.