Streaming potential method at tangential flow of solution has been proposed as a means of measuring the surface potential of hollow fiber membranes. Zeta potential and surface charge for polyacrylonitrile hollow fiber nanofiltration membranes have been measured. The measurements have been made on the membranes of different porosities obtained by partial drying of initially wet membranes. Porosity has been evaluted from the rate of pressure-driven filtration of a solution through membrane pores. An equation has been derived that allows for assessment of the effect of solution flow in the porous layer. It has been shown that more correct values of membrane surface potential can be calculated using this equation.