The zeolitic imidazolate framework-8 (ZIF-8) nanoparticles were synthesized at two different sizes of 80-100 nm (ZIF-8 (Z1)) and 60-70 nm (ZIF-8 (Z2)) and characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) analyses. The synthesized ZIF-8 nanoparticles were used for modification of polyvinylidene fluoride (PVDF) ultrafiltration membranes prepared by the phase inversion technique. The SEM and atomic force microscopy (AFM) analyses were used to monitor the changes in the morphology and surface properties of the bare and ZIF-8 modified membranes. The results of AFM analysis revealed the decrease in the surface smoothness of the modified membranes. The ZIF-8/PVDF mixed matrix membranes presented higher water flux than the bare PVDF membranes owing to their higher porosity and suitable pore size of the ZIF-8 nanoparticles. By addition of 0.1 wt% of ZIF-8 (Z1) and 0.2 wt% ZIF-8 (Z2) nanoparticles, the modified membranes displayed water flux of 310 and 275 L/m(2)h, respectively, with 98 and 76% increase compared with the unmodified PVDF membrane. The results indicated that the membranes modified with larger ZIF-8 particles (80-100 nm) showed higher permeability and flux recovery ratio (FRR %) than those with smaller particles (60-70 nm).