: Microstructure of seed-coat surface on the soybean cv. Enrei was investigated to reveal the structural properties of the side-wrinkled seed (SWS). The microstructure with small depressions, which is frequently observed on the surface of waterpermeable seed coats, was profi led by using a surface-scanning laser microscope. The profi le provides a root mean square length from standardized surface (RMS) calculated via geometrical analysis, which is an orthodox indicator of the roughness on the focused surface. RMS on the center area of the abaxial-side seed coat in the normal (wrinkleless) seed was 1 . 4−1 . 6 μm, whereas that in SWS was signifi cantly reduced to 0 . 9−1 . 1 μm. This suggested that the SWS surface was partly smoother than the normal seed surface. The short-time imbibition tests on the normal seeds and observation of wrinkle induction, demonstrated that, on the abaxial side, low-RMS seed coats tended to wrinkle less than the high-RMS seeds, while on the dorsal side all imbibed seeds wrinkled. Although a similar difference in the seed-coat RMS was observed in the normal seeds, simple moistening-drying process of the seeds provided no evident SWS formation. These results suggest that not only the changes in surface microstructure of seed coat but also some seed-fi lling process may affect SWS formation.