Membranes are widely used in wastewater treatment. In this study, cellulose acetate (CA) (M-w = 52,000) was mixed with polyethylene glycol (PEG) (M = 400) as an additive in N-methyl-2-pyrrolidone as a solvent. The phase inversion method was used for preparation of flat sheet membranes. Effects of CA and PEG concentrations and also coagulation bath temperature on the membrane morphology were discussed in terms of membrane thickness and cross section structure. Pure water permeation flux of the membranes was also measured.