Polyacrylonitrile (PAN) membrane is known as one of the hydrophilic membranes for ultrafiltration. However, the membrane has been preventing from the versatile applications, because the semi-crystalline PAN membranes are so brittle that cannot reuse once the membrane has been dried. The effect of crystalline domains in asymmetric polyacrylonitrile membranes is investigated, when the membranes are annealed in hot water and when the membranes are dried. Asymmetric polyacrylonitrile membranes were prepared via phase inversion process in a water bath and the effect of additive, PVP to the casting solution on the morphology and the water flux and the rejection were investigated. When the membranes were annealed in hot water (80 โข C), the size of pores have been reduced and the water flux also decreased. Using wide angle X-ray scattering (WAXS), the effect of absorbed water on PAN membranes was studied. The absorption of water in PAN membranes mainly occurred through amorphous phase like a plasticizer, and induced the change of crystalline structure. The size of crystallite and the degree of crystallinity have changed when the membrane were annealed in the hot water. When the asymmetric PAN membranes were dried, the moisture also plays a crucial role in transforming the crystalline structures. The kinetics of drying strongly influences the size of crystallite as well as the crystallinity.