Spontaneous formation of microstructures on solid surfaces is an attractive approach for developing functionalized surfaces because it is potentially useful as a facile alternative to conventional micromachining techniques. In this study, we succeeded in obtaining spontaneously formed microwrinkles with a well-ordered stripe pattern. The experimental procedure was very simple, as follows. A poly(dimethylsiloxane) sheet was bent and then soaked in a mixture of sulfuric acid and nitric acid. The sheet was made flat again and washed with water. The wrinkling pattern obtained typically showed a wavelength of 75 mu m and an amplitude of 4.5 mu m. In addition, the wrinkles were observed over a wide area of the sample, 15 mm X 30 mm. Infrared spectra of the sample revealed that the acid solution oxidized the surface of the sheet and formed siloxane bonds, suggesting that the surface was stiffened due to the cross-linkage of the polymer chains by these bonds. Thus, a stiff film was formed on top of the elastic poly(dimethylsiloxane) substrate. Therefore, when the bent sheet was made flat again, compressive stress in the film caused uniform wrinkling due to mechanical buckling instability. Because this wrinkling method does not require any vacuum processes such as metal evaporation, it is potentially advantageous over conventional micromachining methods when a large surface is to be wrinkled and when a wrinkled structure is to be produced using continuous processes.