Abstract: The cumulative protonation constants of ethylenediamine-N,N,N’,N’-tetramethene phosphonic acid(EDTMP,LH_8) and the stability constants of its Sm(Ⅲ)complexes are determined by pH-potentiometry at(37.0±0.1)℃. The experimental data are analyzed with the software package Hyperquad 2000. The first protonation constant lgβ_1=12.90 of EDTMP is taken from literature. The following values are found:lgβ_2=22.69±0.02,lgβ_3=30.54±0.03,lgβ_4=36.93±0.02,lgβ_5=42.15±0.01,lgβ_6=45.02±0.03,lgβ_7=46.14±0.02 for EDTMP, and lgK_(SmL)=22.62±0.02,lgK_(SmLH)=29.93±0.02,lgK_(SmLH_2)=36.18±0.03 for Sm-EDTMP. The speciation studies indicate that EDTMP exists principally in the form of LH_3, and Sm(Ⅲ) in the forms of SmLH and SmL at physiological pH. Plasma thermodynamic equilibrium model simulation shows that SmLH and SmL are the predominant species of ~(153)Sm in blood after intravenous injection of ~(153)Sm-EDTMP. The existence of ~(153)Sm as negatively charged ~(153)Sm-EDTMP complexes in blood could explain why the administrated ~(153)Sm-EDTMP is mainly excreted by kidneys. The complexation of Ca~(2+) by plasma EDTMP is found negligibly small, and lost of blood calcium would not be a problem during the palliative therapy of bone tumor.