In this work the thermodynamic model of Chen and Sun which describes the adsorption of protein on a hydrophobic gel in hydrophobic interaction chromatography process is modified by substitution activity instead of protein and salt concentration in liquid phase. The model is based on two-state equilibrium of protein in solution and adsorbed phase. Also, the effect of salt concentration and type of hydrophobic gel on the amount of protein adsorption is investigated. Finally, the accuracy of model is evaluated by measuring average absolute deviation (AAD) for adsorption isotherm in different salt concentration. The results show that the modified model had high accuracy for prediction the adsorption isotherm in different type of adsorbed and salt concentration.