Polyethersulfone (PES) microfiltration membranes can be produced from PES/polyethylene glycol(PEG)/ solvent systems by dry/wet phase inversion process．Some factors influencing pore size of microfiltration membranes are diseussod including kiads of solvent and the concentration of PEG and PES．The results indicated that the surface morphology and properties of miempomus membranes were determined by the concentration of PEG and PES．and the kind of solvent had little influence on the morphology of the membrane．In such casting solutions an increase of PEG concentration from 70％ to 80％ (by weisht) increased the viscosity of the solution significantly It also changed the size of polymer aggregates and the demixing rate of the casting solution in air and in water

therefore PES membranes having different morphologies were obtained．By adjusting the concentration of PES and PEG

mirofillitration membranes with various average pore size can be produced．