It was found that, by the addition of appropriate hydrophilic nonsolvent in the hydrophobic polymer solution, hydrophobic polymeric membranes with ＇cellular surface＇ structure can be fabricated. ＇Cellular surface＇ is to denote the structure of the membranes having surface pores with a polygonal (nearly hexagonal) pattern. In the present work, the formation mechanism of the ＇cellular surface＇ structure was investigated by visualizing the change on the surface of casting solution during membrane formation. When the hydrophobic polymer solution containing hydrophilic additives was exposed to air containing water vapor, the water vapor was drawn onto the casting solution and resulted in nucleation of emulsion drops spontaneously. The nuclei of emulsion drops grew, coalesced, and ended up with the cellular surface pores when the polymer was precipitated. With the knowledge of the formation mechanism of surface pores. the size surface pores can be tailored by controlling the growth of the nuclei of emulsion drops, either by adjusting the amount of hydrophilic additives or by altering the time period, allowing the nuclei to grow.