The interaction of human serum albumin with divalent nickel ion was studied by equilibrium dialysis, isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC) and circular dichroism (CD) in 30 mmol/L Tris buffer, pH=7.0. There is a set of 8 identical binding sites for nickel binding on the protein at two temperatures of 300 K and 310 K. The cooperativity in the binding is observed at 310 K. The Hill coefficients at 300 K and 310 K are 0.97 and 1.25, respectively. The interaction between nickel ions and HSA is exothermic. A value of －36.5 kJ for enthalpy of interaction (1∶1 stoichiometry) was obtained. The secondary structure of HSA dose not show any change during the binding nickel ions process. However, the tertiary structure of the protein changes, which shows the existence of two natives like states.