Composite nanofiltration (NF) membranes with enhanced structural stability were fabricated and used to concentrate oligomeric proanthocyanidins (OPC) in ethanol solution. The composite NF membranes were prepared by interfacial polymerization of piperazine (PIP) with trimesoyl chloride (TMC), upon the porous supports of polyethersulfone (PES) and polyvinyl formal (PVF) blend membranes. The active layers of composite NF membranes were covalently linked to porous supports owing to hydroxyl groups of PVF upon support surface, which could participate in the interfacial polymerization reaction. The pure water fluxes of the composite NF membranes reached 34.9 L/m2h, while the rejections of Na2SO4 and orange GII were 92.7% and 98.4%, respectively. The enhanced structural stability of the composite NF membranes were confirmed by long-term immersion experiment in ethanol. NF concentration process was considered as a potential alternative to conventional evaporation concentration process for OPC concentration in ethanol solution.