An energy-transfer metal-organic nanoprobe is designed for ratiometric sensing with dual response to both peroxynitrite (ONOO-) and hypochlorite (ClO-). Here, a nanoscale metal-organic framework (NMOF) acts as the energy donor and molecular probe as the acceptor to construct a Forster-resonance energy transfer (FRET) nanosystem. Biocompatible dextran conveniently binds to the NMOF surface through multiple weak coordination interactions to improve water dispersibility and cell uptake. Dextran can also coordinate with the molecular probe with arylboronic acid group, which enables the convenient grafting of molecular probes to the NMOF surface to construct energy-transfer nanoprobes. Because of efficient FRET, the bright blue fluorescence of NMOF is quenched, whereas red emission from the acceptor is enhanced. Upon reacting with ONOO-, the probe departs from NMOF and the fluorescence of NMOF is recovered because of the interruption of FRET. When reacting with ClO-, the phenothiazine moiety in the molecular probe is oxidized into phenothiazine-5-oxide, which leads to more efficient energy transfer and the fluorescence shifts from red to orange. The nanoprobes are also successfully applied to the detection of ONOO- and ClO- in living cells.