Integrally skinned asymmetric polyetherimide (PEI) membranes were prepared by the phase inversion process from casting solution containing dimethylformamide (DNIF) as a solvent and 1,4-dioxane as a cosolvent. Deionized water was used as a coagulation medium in preparing asymmetric membranes. The effect of 1,4-dioxane was investigated by measuring casting solution properties, permeation properties, and membrane structures. Various effects of polymer concentration, evaporation time, and coagulation bath temperature were also studied. Low miscibility of 1,4-dioxane with coagulant (water) resulted in reducing membrane pore size. The molecular weight cutoff values of asymmetric membranes could be controlled by changing the amount of 1,4-dioxane in the casting solution.