The fruiting bodies of Isaria japonica have been traditionally used in Korea to treat cancer. An apoptosis-inducing compound, 4-Acetyl-12, 13-epoxyl -9-trichothecene-3, I5-diol (AETD), was isolated from the methanol extract of fruiting bodies of Isaria japonica Yasuda by bioassay -guided fractionation. The apoptosis of murine bladder cancer cell line (NBT-Ⅱ) by the compound was accessed by propidium iodide staining flow cytometric analysis, and apoptosis-inducing activity at $IC_{50}$ concentration (5 nmol／L) was further confirmed by a nuclear morphological change, a ladder pattern of DNA fragmentation, and an activation of caspase-3. These results indicate that AETD induces apoptosis of NBT-Ⅱ cells via expression of caspase-3.