Water treated cellulose acetate (CA) membrane's surface was studied by atomic force microscopy (AFM). It was observed that when CA membrane (water untouched) was treated with water, the morphology of the surface change was detected by AFM. The roughness parameter of the surface was increased. No significant change was observed on the surface on drying the water treated membrane at room temperature for four days. The results were discussed on the basis that CA membrane contains continuous channels (network pores), which were formed in water-swollen polymer matrix. These water channels are responsible for the rejection of salt in reverse osmosis (RO) phenomenon.