The structure and performance of modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) ultra-filtration membranes prepared from casting solutions with different concentrations of poly(vinyl pyrrolidone) (PVP) were investigated in this study. Membrane properties were studied in terms of membrane compaction, pure water flux (PWF), water content (WC), membrane hydraulic resistance (R-m), protein rejection, molecular weight cut-off (MWCO), average pore size, and porosity. PWF, WC, and thermal stability of the blend membranes increased whereas the crystalline nature and mechanical strength of the blend membranes decreased when PVP additive concentration was increased. The contact angle (CA) decreased as the PVP concentration increased in the casting solution, which indicates that the hydro-philicity of the surface increased upon addition of PVP. The average pore size and porosity of the PVdF-co-HFP membrane increased to 42.82 angstrom and 25.12%, respectively, when 7.5 wt% PVP was blended in the casting solution. The MWCO increased from 20 to 45 kDa with an increase in PVP concentration from 0 to 7.5 wt%. The protein separation study revealed that the rejection increased as the protein molecular weight increased. The PVdF-co-HFP/PVP blended membrane prepared from a 7.5 wt% PVP solution had a maximum flux recovery ratio of 74.3%, which explains its better antifouling properties as compared to the neat PVdF-co-HFP membrane. POLYM. ENG. SCI., 55:2482-2492, 2015. (c) 2015 Society of Plastics Engineers