The ability to synthesize upconversion nanocrystals (UCNCs) with tailored upconversion luminescence and controlled size is of great importance for biophotonic applications. However, until now, limited success has been met to prepare bright, ultrasmall, and monodispersed β-NaYF₄:Yb/Er UCNCs. In this work, we report on a synthetic method to produce monodisperse hexagonal NaYF₄:Yb/Er nanocrystals of ultrasmall size (5.4 nm) through a precise control of the reaction temperature and the ratio of Na⁺/Ln/F. We determined the optimum activator concentration of Er to be 6.5 mol % for these UCNCs, yielding about a 5-fold higher upconversion luminescence (UCL) intensity than the commonly used formula of NaYF₄:30% Yb/2% Er. Moreover, a thin epitaxial shell (thickness, 1.9 nm) of NaLnF₄ (Ln = Y, Gd, Lu) was grown onto these ultrasmall NaYF₄:Yb/Er NCs, enhancing its UCL by about 85-, 70- and 50-fold, respectively. The achieved sub-10-nm core and core-shell hexagonal NaYF₄:Yb/Er UCNCs with enhanced UCL have strong potential applications in bioapplications such as bioimaging and biosensing.