In this paper, asymmetric flat sheet membranes are prepared with polyetherimide (PEI)/N-methyl-2-pyrrolidone (NMP)/nonsolvent (NS, NS = H2O, ethanol (EtOH), glycerin (PG), polyethylene glycol200 (PEG200)) casting solutions by wet phase-inversion process with different approaching ratios. The thermodynamic behavior of PEI/NMP/NS casting solutions is directly described with the modified LCC (linearized cloud point curve) correlation. For PEI/NMP/NS (NS = H2O, PG) casting solutions, the critical approaching ratio alpha* does not exist and the finger-like macrovoids cannot be suppressed completely. But for PEI/NMP/NS (NS = PEG200, EtOH) casting solutions, with the increase of approaching ratio, the membrane morphology changes from finger-like to sponge-like structure at the critical approaching ratio alpha*. For the flat sheet membranes prepared with PEI/NMP/NS (NS = H2O, PG), with the increase of membrane thickness, the transition of membrane morphology from sponge-like to finger-like structure occurs at the critical structure-transition thickness Lc, and the increase of approaching ratio induces the increase of the critical structure-transition thickness Lc and the general sponge-like structure thickness Lgs. But for the flat sheet membranes prepared with PEI/NMP/NS (NS = EtOH, PEG200), when the approaching ratio is no less than the critical approaching ratio alpha*, the critical structure-transition thickness Lc is not obtained and a fully sponge-like structure is prepared. (C) 2010 Elsevier B.V. All rights reserved.