asymmetric polyethersulfone (pes) microfiltration flat sheet membranes were composed by the phase inversion method (pim) and were used as supports. composite membranes were fabricated by coating silicone rubber as selective layer. effect of different concentrations of pes and pdms and different solvent such as nmp, dmf and dms effects as pes solvents and support thickness and different coagulation bath temperature were probed on membrane performance and best one was fixed as base and then some concentration of pvdf, methanol, ethanol were served in some concentration as additives in pes solutions and also methanol and ethanol in water coagulation bath to promote the qualities of membrane for separating oxygen from nitrogen. the best polyethersulfone (pes) membranes structure for o2/n2 separation were made by adding methanol to casting solution and were used as supports of magnetic membranes. composite membranes were fabricated with coating fluidmag-pad on both sides of pes membrane surface and pes membrane coating with pdms and between two layers of this composite membrane to separate of oxygen from nitrogen. permeance and o2/n2 selectivity were evaluated in absence and presentation of external magnetic field. the coil was served for producing external magnetic field. prior to probe the influences of using the external magnetic field which changes the superparamagnetic particles to magnetic ones, the effect of this superparamagnetic surfactant, fluidmag-pad which has large surface area, on membrane performance was investigated. this results conclude from experiments: 1. support and coating concentration can control the permeance. 2. the membranes showed various performances for air separation depending on the coating applied on dense or porous layer. the latter results in superior performance. 3. thickness must be adjusted with two parameters, satisfied permeance and mechanical strength. 4. solvent is selected based on structure that is required, for instance finger-like structure with dense layer is satisfied for air separation. 5. using incompatible polymers to prepare blend membrane, compose channel-like structure that is not useful for air separation whereas increase the permeance as the result of forming dispersed phase from polymer with less concentration. 6. to achieve suitable structure, alcoholic additives were used in casting solution and coagulation bath, solubility parameter plays important role to control membrane structure. 7. the permeance changes have the direct trend with solubility parameter difference between polymer and solvent mixture, for instance when this parameter difference was lowest, lowest permeance was obtained 8. the permeance changes have the reciprocal trend with solubility parameter difference between solvent and nonsolvent mixture, for instance when this parameter difference was lowest, higher permeance was obtained. 9. coagulation bath temperature can verify membrane structure as releasing or fixing stress between the entanglements in casting film, best selectivity obtain in lowest temperature with more permeance. 10. finger-like structure with dense layer has best selectivity and suitable permeance. 11. super-paramagnetic materials such as fluidmag-pad are able to promote the membrane performances via expanding the active area as the coating layer. 12. the membranes showed various performances for air separation depending on the coating applied on dense or porous layer. the latter results in superior performance. 13. coating pdms on bottom layer and fluidmag-pad on top layer of pes asymmetric membrane resulted in higher performance.