Abstract: A thermodynamic equilibrium model, consisting of stability constants of major metal-ions and low molecular weight molecules ligands in human blood plasma, are made to analyze the constituent concentrations of Sm-EDTMP in vivo. The simulation analysis with computer program pack Hyperqud 2 000 indicats that the ligand EDTMP existes mainly in the forms of EDTMPH2-4 and EDTMPH3-5 in vitro at physiologic pH value and the net negative charge of ligand decreased with lowering pH. So, higher pH is favorable to form and stabilize Sm-EDTMP. The research results about Sm-EDTMP show that the complex existe mostly in the forms of Sm-EDTMP-5, Sm-EDTMPH-1-6 and Sm-EDTMPH-4 in vitro at pH 7. 4 which imply the complex will excrete mostly by kidney and be less harmful to liver. Sm-EDTMP study results with thermodynamic equilibrium model in blood plasma show that the complex is with -4 charge in the form of Sm-EDTMPH-4. The large negative charge of Sm-EDTMP provides, the power of chemisorption to HA.