The relationship between the bound chloride and the free chloride in concrete is very important to the durability of concrete structures. The high performance concrete (HPC) with fly ash was prepared and the specimens were tested by ASTM C1202 in this paper. The concentrations of the total and the free chloride in HPC specimens were determined after the experimentation, by which the concentration of the bound chloride was calculated. The results show that the relationship between the free chloride and the bound chloride in HPC can be described by Freundlich adsorption isotherm, and the coefficient α and β in the equation are affected by initial bound abilities and bound potentialities respectively. The bound potentiality of HPC decreased inversely with the amount of replacement of fly ash.