The addition of nonsolvent in the casting solution can suppress the formation of macrovoids in polymeric membranes if the added nonsolvent can induce polymer gelation. FTIR microscopy reveals that the formation of PMMA gels would result in a great reduction of the nonsolvent flux from coagulation bath to the casting solution, which can well account for the disappearance of macrovoids. The associated gelation mechanism was also investigated. After the phase separation, induced by the added, nonsolvent, unvitrified polymer gels were obtained if the polymer-rich phase has a suitable elasticity. Although the unvitrified gels are not thermodynamically stable, they can be sustained long enough to affect the membrane morphology during membrane formation. The gelation boundary can be determined by the falling ball experiment, and the equality of the storage and loss moduli in the polymer solution can be used to describe the required rheological property for the gels.