We present detailed information on the fabrication of free-standing ultrathin porous alumina membranes (PAMs) with controllable thickness of 100–1000 nm. The mechanism of the ultrathin PAM formation has been revealed by a combination study of current–time characteristics and microstructure images. At the beginning of the anodization, V-shape nanopores can be observed due to the alumina formation in both the sidewalls and the barrier layers. As a result of the applied electric field effect, part of the alumina in the sidewalls is dissolved, the nanopores gradually become regularly U-shape and finally grow steadily. Ultrathin PAMs with controllable thickness and morphology have been realized by changing the anodization time and the current density. Furthermore, an improved method has been demonstrated to obtain free-standing ultrathin PAMs by removing unoxided aluminium through a nontoxic mixture solution of saturated CuSO₄ and HCl.