An asymmetric microfiltration membranes were prepared with polysulfone by an immersion precipitation phaseinversion method. Microfiltration membranes were prepared by polysulfone/N-methyl-2-pyrrolidone/polyvinylpyrrolidone/phosphoric acid casting solution and water coagulant. The vapor induced phase inversion method was used to prepare themembranes. The pore size and the morphology were changed by the phosphoric acid additive, the temperature of castingplate and the exposure time at the relative humidity of 74%. The morphology of membranes was investigated by scanningelectron microscopy and microflow permporometer. By the addition of the phosphoric acid additive in the casting solution,the morphology of the prepared membranes were changed from a dense sponge structure to a loose asymmetric spongestructure. Due to the addition of catalytic amount of phosphoric acid to NMP casting solution, the mean pore size increasedalmost 0.2 μm and the water flux increased about 3,000 LMH. The temperature of casting plate and exposure time had aapparent effect on the skin layer structure and the pore size and the porosity of the membrane.