Reactive　oxygen　species　(ROS)　contribute　to　cellular　redox　environment　and　serve　as　signaling　molecules.　Excessive　ROS　can　lead　to　oxidative　stress　that　are　involved　in　a　broad　spectrum　of　physiological　and　pathological　conditions.　Stem　cells　have　unique　ROS　regulation　while　cancer　cells　frequently　show　a　constitutive　oxidative　stress　that　is　associated　with　the　invasive　phenotype.　Antioxidants　have　been　proposed　to　forestall　tumor　progression　while　targeted　oxidants　have　been　used　to　destroy　tumor　cells.　However,　the　delicate　beneficial　range　of　ROS　levels　for　stem　cells　and　tumor　cells　under　distinct　contexts　remains　elusive.　Here,　we　used　Drosophila　midgut　intestinal　stem　cell　(ISCs)　as　an　in　vivo　model　system　to　tackle　this　question.　The　ROS　levels　of　ISCs　remained　low　in　comparison　to　that　of　differentiated　cells　and　increased　with　ageing,　which　was　accompanied　by　elevated　proliferation　of　ISCs　in　aged　Drosophila.　Neither　upregulation　nor　downregulation　of　ROS　levels　significantly　affected　ISCs,　implicating　an　intrinsic　homeostatic　range　of　ROS　in　ISCs.　Interestingly,　we　observed　similar　moderately　elevated　ROS　levels　in　both　tumor-like　ISCs　induced　by　Notch　(N)　depletion　and　extracellular　matrix　(ECM)-deprived　ISCs　induced　by　p-integrin　(mys)　depletion.　Elevated　ROS　levels　further　promoted　the　proliferation　of　tumor-like　ISCs　while　reduced　ROS　levels　suppressed　the　hyperproliferation　phenotype;　on　the　other　hand,　further　increased　ROS　facilitated　the　survival　of　ECM-deprived　ISCs　while　reduced　ROS　exacerbated　the　loss　of　ECM-deprived　ISCs.　However,　N-　and　mys-depleted　ISCs,　which　resembled　metastatic　tumor　cells,　harbored　even　higher　ROS　levels　and　were　subjected　to　more　severe　cell　loss,　which　could　be　partially　prevented　by　ectopic　supply　of　antioxidant　enzymes,　implicating　a　delicate　pro-surviving　and　proliferating　range　of　ROS　levels　for　ISCs.　Taken　together,　our　results　revealed　stem　cells　can　differentially　respond　to　distinct　ROS　levels　under　various　conditions　and　suggested　that　the　antioxidant-based　intervention　of　stem　cells　and　tumors　should　be　formulated　with　caution　according　to　the　specific　situations.