The effect of molecular weight of polydiallyldimethylammonium chloride (PDADMAC) (<100 kDa, 200-350 kDa, 400-500 kDa) in the aqueous solution used to prepare the intermediate layer on the surface of PSF ultrafiltration membrane-substrate on the structure and performance of thin film composite membranes for nanofiltration obtained via interfacial polymerization was studied. It was found that adsorption of PDADMAC on the surface of PSF membrane-substrate and increase of PDADMAC molecular weight yields the decrease of pore size, porosity, water contact angle and surface roughness parameters. It was shown that application of PDADMAC intermediate layer prior formation of polyamide selective layer via interfacial polymerization results in generating thinner, highly hydrophilic, more positively (less negatively) charged polyamide selective layer with higher cross-linking degree, smaller pore size and lower surface roughness parameters. It was revealed that increase of PDADMAC molecular weight up to 400-500 kDa yields the rise of polyamide selective layer thickness and surface roughness as well as the change of selective layer structure from the structure with globules to the crumbled and folded one. Thin film composite membranes with PDADMAC intermediate layer demonstrated higher water permeance (8.6-11.4 L/(m2·h·bar) compared to the 4.2 L/(m2·h·bar) for the reference membrane), higher rejection of MgSO4 (93-98% compared to 92% for the reference membrane), higher rejection of PEG-300 (98% compared to the 96% for the reference membrane) and dyes with large positively charged organic ion, higher Mg2+/Li+ separation factor (18-38 compared to 8 for the reference membrane) and better antifouling performance.