In order to interpret the formation mechanism of asymmetric polysulfone ultrafiltration membranes from equilibrium thermodynamics and kinetics points of view, we changed the power of the precipitant by adjusting the solvent content in precipitant (v₂) and investigated its effect on the binodal curve and precipitation path, and hence, on membrane performance. Experimental cloud point (binodal) curves were found to shift toward the polymer-precipitant axis in a ternary phase diagram with v₂. We also calculated the ratio of precipitant inflow and solvent out-flow, J_l/J₂, for the prediction of precipitation path, where the ratio increased with increasing v₂ and was unaffected by increasing polymer content in casting solution(φ₃). As the value of v₂ became higher, the skin layer of membrane formed became more porous. yielding the decreasing rejection efficiency. However, pure water flux decreased with increasing v₂ because of the formation of dense supporting layer, as evidenced by the morphological observations using the SEM.