Abstract: In this paper a new formula of activity coemcicnt of electrolytes has been proposed in terms of Stokes and Robinson Theory (1948). The new formula has all of the advantages of Stokes and Robinson formula (1948), but does not contain osmotic coemcicnt(φ). Futhermore, the new formula is applicable to single—solvent and mixed—solvent electrolyte systems. The achvity coefficients of 22 electrolyte—containing systems are calculated by the new formula. The results have shown good accuracy. The mean or relative deviation of every processed system is less than 1%.