The anionic long circulating liposomes (ALCLs) prepared in previous studies enabled high loading efficiency, small particle size, and good stability. In addition, the uptake by tumor cells and delivery of radioiodinated antisense oligonucleotides (ASON) to MCF-7 breast cancer cell line was improved. The liposomes also displayed valuable pharmacologic properties characterized by a long half-life, slow clearance, and a significant area under the concentration–time curve (AUC) in vivo. This study emphasizes the contribution of ALCLs to the anti-tumor effect of ASON and radiation therapy. Our results show that ALCLs improve biodistribution and delivery of radioiodinated ASON in tumor bearing rats, which is characterized by the suppression of tumor growth, decreased bcl-2 expression, and increased tumor cell necrosis in the mammary tumor.