A systematic study of the zeta potential and isoelectric point of biomolecules such as BSA, amylase, invertase and phenylalanine has been performed in various salt solutions for 0.001 M and 0.1 M ionic strenght. Chloride salts; KCl, NaCl, CaCl2, and MgCl2 and potassium salts; KCl, KNO3, K2CO3 and K2SO4 were used to test the effects of cations and anions on the zeta potential of biomolecules, respectively. The absolute zeta potential of biomolecules decreased with increasing ionic strenght; divalent ions had a profound influence on reducing zeta potential than monovalent ions. The cations were less effective at low pH and became more effective at high pH for both of ionic strength. Anions possessed a more potent effect on the zeta potential at high and low pH except for 0.1 M salts concentration at high pH. In general, isoelectric point of biomolecules changed with ionic environment. In addition, the effects of ions on the zeta potential of biomolecules were also interpreted for Hofmeister series.