Endonuclease G (EndoG) is a mitochondrial protein that is released from mitochondria and relocated into the nucleus to promote chromosomal DNA fragmentation during apoptosis. Here, we show that oxidative stress causes cell death defects in C. elegans through an EndoG-mediated cell death pathway. In response to high ROS levels, homodimeric CPS-6－the C. elegans homolog of EndoG－is dissociated into monomers with diminished nuclease activity. Conversely, the nuclease activity of CPS-6 is enhanced and its dimeric structure is stabilized by its interaction with the worm AIF homolog, WAH-1, which shifts to disulfide cross-linked dimers under high ROS levels. CPS-6 thus plays an unexpected role in acting as a ROS sensor to regulate the life and death of cells. Modulation of the EndoG dimer conformation could present an avenue for prevention and treatment of diseases resulting from oxidative stress.