Provided is a reagent composition for measuring glycated hemoglobin to diagnose the presence or absence of diabetes and a method of measuring glycated hemoglobin using the same, and more particularly is a reagent composition for measuring glycated hemoglobin, the composition including a dye-encapsulated silica nanoparticle-boronic acid, and to a method of measuring glycated hemoglobin using the same. In the reagent composition for measuring the glycated hemoglobin, since a dye is encapsulated in silica nanoparticles, the inherent absorption wavelength of the dye is not affected by pH and the composition has excellent stability even when stored for one month or more.