Two microfiltration membranes made of polyvinilidene fluoride (PVDF) and polyethersulfone (PES) of the same nominal pore size have been modified by photo-initiated graft copolymerisation of quaternary salt of dimethylamino ethyl methacrylate (qDMAEM). Atomic force microscopy (AFM) has been used to characterise chan(yes in surface morphology with variation of the degree of polymer grafting and therefore the process of modification. Comparative analysis of pore size distribution (PSD) of the initial and modified membranes has revealed differences in the modification process. Although polymer grafting resulted in PSD shifting towards smaller pore sizes., modification of PES membrane narrowed PSD, whereas modified PVDF showed wider PSD. This was attributed to different free-radical density for photo-reactive PES and more inert PVDF with possible contribution of the surface roughness.