A new two-dimensional zeolitic imidazolate framework with leaf-shaped morphology (ZIF-L) was incorporated into polyethersulfone (PES) ultrafiltration membranes to investigate how the ZIF nanoflakes affect functional membrane properties. The membranes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and contact angle goniometry. The water permeability and molecular weight cut-offs (MWCO) of membranes were also determined under constant pressure filtration. Membrane fouling resistance was characterized under constant flux operation using bovine serum albumin (BSA) as foulant. The modified UF membrane with 0.5% ZIF-L loading showed around 75% increase in water flux without greatly affecting the MWCO. Also, the same membrane showed almost twice the fouling resistance improvement against BSA with more than 80% water flux recovery. The improvement was due to the combined effect of the lower zeta potential of the modified membrane, increased hydrophilicity and reduced surface roughness, which made the attachment of BSA protein on the membrane surface more difficult. These results demonstrate that the addition of 2-dimensional ZIF-L nanoflakes is effective for improving polymer membrane fouling resistance and water flux.