The effect of 2-methoxy ethanol (2-ME) nonsolvent as an additive in a casting solution on the morphology of microfiltration menoranes was studied. Microfiltration polymer membranes were prepared by using polyethersulfone (PES)/N-methyl-2-pyrrolidone (NMP)/2-ME casting solution and water coagulant. The two successive processes of the vapor-induce phase inversion (VIPI) and the nonsolvent-induced phase inversion (NIPI), which followed VIPI, were used to prepare the membranes. The pore size and the morphology were changed by the nonsolvent additive and the exposure time at the relative humidity of 74%. The pore size was then measured by a capillary flow porometer. By increasing the concentration of the nonsolvent additive in the casting solution, the morphology of the prepared membranes were changed from a macrovoid structure to a sponge structure. Also relative humidity, exposure time, the change of the concentration of the additive, the viscosity of casting solution and the temperature of coagulation bath had a pronounced effect on the pore size and the porosity of the membrane.