To evaluate the in-vivo and in-vitro stability of methylene diphosphonate $(MDP)-^{99m}Tc$, relative ligand exchange rates in phosphate buffer between $MDP-^{99m}Tc$ and human serum albumin (HSA), and between pyrophosphate$(PYP)-^{99m}Tc$ and HSA have been measured. Gel permeation chromatography has also been carried out to estimate relative rates of ligand exchange between polysaccharide and the $^{99m}Tc-bone$ agents. The in-vitro stability was further correlated with its specific radioactivity. The results indicated that the $MDP-^{99m}Tc$ is more stable than the $PYP-^{99m}Tc$ but less stable than $MIDA-^{99m}Tc$, $HIDA-^{99m}Tc$, and $DTPA-^{99m}Tc$ etc. in refering to other data. In stability point of view, $MDP-^{99m}Tc$ is considered to be a better bone agent than $PYP-^{99m}Tc$ since $MDP-^{99m}Tc$ can be accumulated at bone by a smooth transfer of $^{99m}Tc$ to hydroxyapatite, whereas in case of $PYP-^{99m}Tc$, most of the $^{99m}Tc$ is transferred to HSA before arriving to the hydroxyapatite.