Studies on streaming potential (SP), an important electrokinetic phenomena, is chiefly within the scope of microfiltration (MF) and ultrafiltration (UF) membrane processes based on the Helmholtz-Smoluchowski (HS) equation. However, the HS equation has some limitations, which is an adequate cause of the less study of SP for nanofiltration (NF) membranes. In this paper, experiments of SP for two commercial NF membranes (NTR7250 and ESNA 1-K) have been carried in four electrolytes (KCl, MgCl2, Na2SO4 and MgSO4) solutions. We proposed a new modified HS equation to analyze the experimental data and predict the electrical properties of NF membranes. Results show that the modified HS equation is more applicable than the classical HS equation in the evaluation of surface electrical properties of NF membranes. The established strategy to evaluate the SP of two typical NF membranes in the paper has important significance for other membranes processes.