We have observed single isolated faulted-half (F) of DAS structure in unreconstructed regions of Si(111) remained after rapid quenching to 380°C, by using scanning tunneling microscopy. We have observed even-sized F of 6×6, 8×8, 10×10 and 12×12 during the size-changes of odd-sized F. The odd-sized F appearing in the size-changes is always irregular-type. We proposed structural models of the even-sized and irregular odd-sized F. As a whole, we observed the size-changes of single F: irregular 5×5-F↔6×6-F↔ irregular 7×7-F↔8×8-F↔irregular 9×9-F↔10×10-F↔irregular 11×11-F↔12×12-F↔irregular 13×13-F . From these results, we proposed the mechanism of DAS domain formation.