Tetras (tetra-arsenic oxide, As(4)O(6)) is a derivative of arsenic used in Korean traditional medicine for the treatment of cancer, but its mechanism remains largely undefined. Recently, a similar arsenic derivative, diarsenic trioxide (As(2)O(3), ATO), has been shown to mediate anti-tumor activity, therefore reigniting interest in the therapeutic effect of arsenic compounds. Here we report that Tetras can effectively mediate an anti-vascular effect on tumors, leading to delay in tumor growth and increased survival. Our study demonstrates for the first time the potential use of Tetras as a radiation therapy enhancement agent for solid tumors. These findings reveal an unappreciated role of Tetras in cancer therapy and its potential application to radiotherapy in achieving local tumor control.