We report the design, synthesis, and evaluation of novel 5-fluorouracil (5FU) prodrugs <b>1a</b>,<b>1b</b> that are efficiently activated by the high level of reactive oxygen species (ROS) in cancer cells. Prodrugs <b>1a</b>,<b>1b</b> selectively kill cancer cells over normal cells and are well-tolerated in mice. The strategy described herein can extend application of chemotherapeutic drugs.