A new aromatic diamine, 3,5-diaminobenzoylpiperazine (3,5-DABP), was synthesized from 3,5-diaminobenzoic acid and 1-formyl piperazine. The structure of 3,5-DABP was identified by FT-IR spectra and H-1 NMR spectra. With 3,5-DABP as aqueous monomer and trimesoyl chloride (TMC) as organic monomer, thin film composite (TFC) nanofiltration membranes were prepared by interfacial polymerization technology. The salt rejection order of these TFC membranes is Na2SO4>MgSO4>MgCl2>NaCl. This sequence indicates that the membranes are negatively charged.