It was observed that rare earth ions (Nd3+, Sm3+, Eu3+, Gd 3+, Tb3+) have significant quenching effects on the fluorescence of anti coagulation factor I (ACF I). The results of the fluorescence titration of ACF I with rare earth ions demonstrate that ACF I has two RE3+-binding sites, and the rare earth ions and Ca2+ bind to ACF I competitively in the two si milar sites. The association constants K1 and K2 of ACF I with each rare earth ions (Nd3+, Sm3+, Eu3+, Gd3+, Tb3+) are close to each other, which indicates the structural similarity of the two bindi ng sites in ACF I. Although the ionic radii of Nd3+, Sm3+, Eu3+ , Gd3+ and Tb3+ are different, both their K1 and K2 ar e similar, respectively. This reveals the conformational flexibility of the two binding sites in ACF I, which offers a possibility for Ca2+ to take p lay in the inducing conformational changes of ACF I and the promoting the bindin g of ACF I with activated coagulation factor X.