Microporous Nylon-66 polymeric membranes were prepared by immersion–precipitation from a ternary system, water/formic acid/Nylon 66. Depending upon the precipitation conditions, membranes with morphologies that reflect the sequence of liquid–liquid demixing (as characterized by cellular pores) and crystallization (as characterized by crystal particles) events during the course of precipitation were obtained. The details of the membrane morphologies were disclosed using a low voltage field emission scanning electron microscope (FESEM) at very high resolutions. In particular, nano-scale fine structures such as dendritic crystal elements, nano-pores, nano-grains, branching lamellae, etc., which were rarely presented in the membrane literature. Wide angle X-ray diffraction analyses indicated that Nylon-66 crystallized into ‘α’ structure in all prepared membranes. Crystallinities were determined by appropriate deconvolution of the diffraction peaks. The results indicated that membranes prepared by a well-dissolved casting dope had a somewhat higher crystallinity than those prepared by incipient dopes being in metastable states with respect to crystallization. This observation was confirmed by Fourier transform infrared spectroscopy and DSC thermo analyses.