Decreasing the thickness of cast polymer solution in a membrane-forming system may lead to structure transition from finger- to spongelike structure. The shrinkage measurement of the cast poly(acrylonitrile) solutions reveals that the maximum value of shrinkage takes place when the macrovoids disappear from the structure. The solute rejection properties of membranes also show a different behavior between below and above the structure transition thickness. This experimental finding is explicable on the grounds of porosity variation of the membrane skin. The scanning electron micrograph of a cross-section membrane shows a nodular structure near the precipitation-bath interface, the structure of which is a result of spinodal demixing that is influenced by varying the thickness of the cast polymer solution. With the cast solution being regarded as two layers including it very thin top layer and a sublayer, closely packed spheres of polymer are considered for discussion of the thickness dependence of the structure evolution of the membranes.