We report in situ observation of wrinkles formation and evolution of Si nanoribbons with finite length on elastomeric substrate via white light interferometer. The wrinkle originates from the middle of the nanoribbon, propagates symmetrically to the two ends, and finally reaches the stable configuration. The wavelength and amplitude will increase abruptly when the released strain exceeds the critical value. The interface interaction between Si nanoribbons and elastomeric substrate plays the key role for wrinkles formation. We gratefully acknowledge the support from National Natural Science Foundation of China (Grant Nos. 10902059, 90816007, 10820101048, and 10832005) and Foundation for the Author of National Excellent Doctoral Dissertation of China (FANEDD) (No. 2007B30).