Membranes with suitable nanopores and tunable charge characteristics are highly desired for their good application prospects in specific membrane-based separations. This study focused on the synthesis of polyamide (PA) composite membranes with tunable pore size and perm-selectivity. Trimellitic anhydride chloride (TAC) with an anhydride group, was introduced as the co-reactant of trimesoyl chloride (TMC) to form PA film via interfacial polymerization with m-phenylenediamine on the surface of the polysulfone substrate membrane. Due to the chemistry character difference (the number of functional group and acylation activity) between TMC and TAC, the structure and physico-chemistry properties of the PA film could be tailored through the control of TAC concentration. As a result, the nanopore radius of the membrane varied from < 0.32 to 1.14 nm, along with the improved water permeability from 2.3 to 9.8 L m(-2) h(-1) bar(-1) and selectivity of Na2SO4/NaCl from 2.2 to 4.9. This study provided a facile and effective approach for the fabrication of tailor-made membrane with tunable nanopore and ion selectivity designed at a molecular level.