Network nanoporous poly(vinylidene fluoride-co-hexafluoropropene (PVDF–HFP) membrane formation mechanism via poly(methyl methacrylate) (PMMA) leaching from PVDF–HFP/PMMA blend film was studied. The blends of PVDF–HFP with PMMA comprise miscible amorphous phase much larger in quantity than those of poly(vinylidene fluoride) (PVDF) with PMMA. The residual PVDF–HFP crystalline phase which may present in very small fraction was characterized by SAXS, which permitted estimation of the critical composition of phase boundary of blend. The FESEM micrograph revealed the network morphology of the membranes being composed of interconnected tiny sheaves of recrystallized PVDF–HFP. Nanogelation mechanism is proposed to describe the crystallization of disentangled PVDF–HFP chain segments under spatial confinement during PMMA leaching from amorphous solution phase in blend film. The crystallinities of membranes determined by DSC are found to be consistent with the theoretically calculated values. Present research suggested a novel approach for the formation of network nanoporous membranes from semicrystalline polymers.