Nanodiamond (ND) particles of different size (10, 250, and 500 nm) were used as filler materials for preparation of polysulfone-nanodiamond (Psf-ND) mixed-matrix membranes (MMMs), with loading of each ND particle varying from 0.1 to 2 wt% of polymer. Control Psf and MMMs were irradiated up to a γ-radiation dose of 1000 kGy. The effect of ND size was investigated on morphology, surface and mechanical properties, and performance of membranes in γ-radiation environment. The membranes were characterized by surface and cross-section SEM images, AFM, water contact angle studies, GPC, UTM, pure water permeability, and solute (PEO, 100 kDa) rejection measurements. The 10 nm sized ND enhanced the stability and performance of MMM up to 0.5% loading, while higher loading of it led to agglomeration of ND, and in-turn the deterioration of membrane performance. ND with larger particle size (250 and 500 nm), though dispersed homogeneously in the membrane matrix, but resulted in interfacial defects between ND and Psf host membrane matrix. The MMM with an optimum 0.5% loading of ND10 particles offered ~5 times enhanced radiation stability, as compared to that of control Psf membrane. This is the first of its kind study reported herein that provides an insight into the effect of ND particle size in altering the radiation resistance behaviour of MMMs. © 2020 Elsevier B.V.