Microporous Nylon-66 membranes were prepared by non-solvent induced phase separation (NIPS) from the Water/1-Butanol/Formic acid/Nylon66 quaternary system. Water, as a strong non-solvent for Nylon-66, was employed to enhance polymer crystal nucleation in the casting dope, while 1-butanol was adopted as a soft coagulant whereby liquid–liquid demixing mechanism can be sufficiently suppressed. By gradually increasing the water content in the dope, the effect of nucleation density on the morphology of the membrane was clearly manifested. While low water content (< 2.5%) dopes resulted in membranes consisting of large full spherulites, high water content (> 7.5%) dopes gave rise to symmetric bi-continuous membranes composed of small interlocked, stick-like crystallites intertwining with continuous channels of micropores. Water permeation flux and tensile strength of the membranes were measured and correlated with the porosity, pore size, and membrane morphology. In addition, X-ray diffraction (XRD) and Fourier Transform Infrared-Attenuated total reflection (FTIR-ATR) analyses indicate that the membranes contained α-type crystals with a crystallinity of ~ 38%, consistent with that determined from Differential Scanning Calorimetry (DSC). The later method also showed that all membranes have a similar crystal melting behavior with Tm ~ 265 °C.