Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted extensive interest in bio-applications due to their unique optical properties by converting near infrared excitation to visible emission. Here, we show that NaGdF₄:48% Yb³⁺,2% Er³⁺ upconversion nanoparticle arrays could be used as a high-sensitive, reusable, and stable device for detecting glycosylated hemoglobin (HbA1c), based on the luminescent resonance energy transfer (LRET) from UCNPs to HbA1c. Importantly, the upconversion nanoparticle arrays present a much higher sensitivity to the concentration variation of HbA1c than the direct detection in the solution, which is ascribed to the extinction effect of the arrays.