Abstract: Electromotive force measurements of the cell, Na-ISE|polyelectrolyte( m_p(mono) ),NaCl( m_a)|AgCl,Ag,are carried out at the different molalities of NaCl in aqueous poly(diallydimethylammonium chloride)(PDADMAC)and poly(anetholesulfonic acid, sodium salt)(PAS) solutions at 308.15 K. The mean activity coefficients of NaCl in aqueous polyelectrolyte solutions are determined by using the standard cell potential which are obtained from the measurements of the electromotive force of cell,Na-ISE| NaCl( m )|AgCl, Ag, combining with the Pitzer model for electrolyte solution.At different concentrations of PDADMAC solution, the mean activity coefficients of NaCl decrease monotonically as the concentrations of NaCl increase.However, for the aqueous solutions of PAS, there is a maximum value as the concentrations of NaCl increase.