A near-infrared (NIR) responsive DNA probe has been developed for DNA hybridization detection with characteristics of high efficient Förster resonance energy transfer (FRET) from small (∼10 nm) upconversion nanoparticles (UCNPs) to nucleic acid stain SYBR Green I (SG, a specific intercalator of double-stranded DNA). The positively charged UCNPs were prepared and facilely attached with single DNA strands by coordination interaction, which acted as probes to detect the DNA targets in the presence of SG. This probe possesses not only the superiority of the NIR-excitation nature of UCNPs which could minimize the autofluorescence background from biomolecules and the photodamage to biological specimens, but also the advantages of facile preparation and high FRET signals. The FRET efficiencies could increase significantly from 2.6% to 12.5% and then to 26% when the size of UCNPs reduced from 94 nm to 30 nm and to 10 nm, respectively. The 10 nm-UCNP-based DNA probe could reach a lower detection limit of complementary ssDNA2 at 3.2 nM and three-base mismatched ssDNA2-M3 at 7.6 nM. The high sensitivity and selectivity of the probe may endow the system with great potential in fluorescence-based biosensing under the irradiation of NIR light.