The tri-peptide sequence of arginine-glycine-aspartic acid (RGD) can specifically bind to the integrin αvβ3 receptor. Accordingly, a variety of radiolabeled RGD-based peptides have been developed for non-invasive imaging of integrin αvβ3 expression via single photon emission computed tomography (SPECT)or positron emission tomography (PET). Among all the RGD radiotracers studied, several RGD monomers have been investigated in clinical trials, and the preliminary results demonstrated specific imaging of various types of tumors, and the tumor uptake correlated well with the level of integrin αvβ3 expression. Recently, several RGD dimeric peptides with PEG linkers have been studied. The new types of RGD peptides showed much higher in vitro integrin αvβ3-binding affinity than the single RGD tri-peptide sequence, and importantly, they exhibited significantly increased tumor uptake and improved in vivo kinetics in animal models. As a representative, 99mTc-3PRGD2 could be easily prepared and exhibited excellent in vivo behaviors in animal models. No adverse reactions are observed in animal models to date.