The interaction of Cu(II) and human serum albumin (HSA) or bovine serum albumin (BSA) at physiological pH is studied by equilibrium dialysis. The successive stability constants are obtained by non-linear least square methods fitting Bjerrum formula. For both the Cu(II)-HSA and Cu(II)-BSA systems, the order of magnitude of K1 and K2 was found to be ≈104 mol 1· dm3. There are about twenty stoichiometry binding sites found in one HSA or BSA molecule. They can be divided into two or three sets. Results of equilibrium dialysis experiments suggest that there exists one strong metal binding site in both Cu(II)-HSA and Cu (II)-BSA. It is the imidazol group nitrogen atoms of His3 that are primarily concerned with copper binding site. After reaching dialysis equilibrium, there is the interaction among the different binding sites, the values ofK all deviate from the simple statistical effect except forK 1 and K2 in both Cu(II)-HSA and Cu(II)-BSA systems, and the positive cooperative effect is found.