Numerous peptides that bind to a given target have been selected by phage display technology. However, some peptides isolated to date do not bind with high affinity to tumor or organ sites, even peptides were selected in vivo. Therefore, the biodistribution of ^99mTc-labeled filamentous phage peptide library via MAG3 (mercaptoacetyltriglycine) were investigated to gain a better understanding of phage circulation in vivo. The experimental results showed that the liver and spleen were the organs of the greatest accumulation, while heart, muscle, pancreas and brain retained less radioactivity. In opposite to other tissues and organs, the radioactivity in stomach, intestine and bone gradually went up with time. The clearance of ^99mTc-labeled phage in blood was very fast from 5 min to 30 min and then slowed down. When phage in vivo circulated at enough long period of time, some phage particles could extravasate in some organs or tissues and internalized there. In conclusion, the circulation time of phage in vivo should be experimentally determined beforehand according to the targeted organs and the specific location of target peptides in order to panning a peptide with high specificity and affinity to that target.