Abstract: A radioiodinated peptide of arginine-arginine-leucine(RRL) for tumor angiogenic endothelium imaging was designed and synthesized. Radioiodination of the RRL peptides was performed by the chloramine-T method. 131I labeled peptides was injected into the nude mice bearing human prostate carcinoma via tail vein. Biodistribution results in vivo was obtained. HPLC of the crude product yielded a main peak containing 99.56% of the absorbance at 220nm. The 131I labeling rate of RRL peptides was about 60%. The radiochemical purity was 96.5%. The radiochemical purity of the labeled compound remained 90.3% at 24h in human blood serum at 37?C. In the biodistribution studies, the 131I labeled RRL peptide accumulated in the tumor to a higher level than in the other organs.