In this paper, the effects of γ-butyrolactone (GBL) weight ratio (wGBL) and membrane thickness on the formation of asymmetric flat sheet membranes prepared with P84 (BTDA-TDI/MDI co-polyimide)/N-methyl-2-pyrrolidone (NMP)/GBL casting solutions are investigated. With the increase of membrane thickness, the transition of membrane morphology from sponge-like to finger-like structure occurs at critical structure-transition thickness Lc. Lc and the general sponge-like structure thickness (Lgs) increase with wGBL. For 20 wt.% P84/NMP/GBL casting solution, the membrane morphology changes from finger-like to sponge-like structure at the critical weight ratio of GBL (w⁎ = 0.69). The membrane morphology and performance of hollow fibers spun with various wGBL are observed. Compared with the hollow fiber membranes made of 18 wt.% P84/NMP/GBL dope solution with wGBL = 0.75, the hollow fiber membranes spun with wGBL = 0.25 present a higher permeation flux and a larger MWCO. As wGBL increases from 0.25 to 0.75, the membrane morphology transfers from finger-like to sponge-like structure. An increase in shear rate shifts the rejection curves towards left, and lowers the MWCO of hollow fiber membranes. For hollow fiber membranes spun with wGBL = 0.75, a relatively high permeation flux and a large MWCO are obtained by the wet spinning process.