Polymeric ultrafiltration membrane formed via phase inversion by immersing polymer solution into nonsolvent bath was simulated with Monte Carlo method applying polymer bond fluctuation lattice model. In the simulation, the exchange algorithm using probability density has been employed for the first time at the boundary to simulate the open and wide diffusion layer in coagulation bath. The simulation focuses on the process of phase inversion of polymer solution from the interface to the inner by spinodal decomposition. It was found that the diffusion behavior of solvent obeys Fick's law and the apparent diffusion coefficient of nonsolvent has exponential relationship to the concentration of polymer solution and operation temperature. The asymmetrical structure from island-like phase to continuous phase comes from the mechanism transition of phase separation from spinodal decomposition to nucleation growth induced by the mutual diffusion of solvent and nonsolvent. (C) 2011 Elsevier B.V. All rights reserved.