A novel ligase-assisted signal-amplifiable DNA detection scheme is demonstrated based on upconversion nanoparticles. Target DNA serves as a template for two designed DNA probes, one of which covalently attached to upconversion nanoparticles, to be joined into a single DNA strand by ligase. The resulting long DNA is able to form a hairpin loop structure, the number of which is amplified by thermal cycling. Energy transfer occurs between the upconversion nanoparticles and the intercalating dyes residing in the stem portion of the hairpin loop, and is monitored to quantify the amount of the initial target DNA. The method displays high sensitivity and specificity for DNA detection.