A single-step synthesis for monodisperse and hexagonal-phase (β)NaYF4:Yb, Er upconversion nanophosphors (UCNPs) with a consistent hexagonalprism shape in the size range from 18 to 200 nm was achieved. The kineticmechanisms for the particle phase transition and growth were examined. Theβ-UCNPs were obtained via co-thermolysis of trifluoroacetate precursors in octadecene(ODE) with combined ligands of oleic acid (OA) and trioctylphosphine (TOP). Theexperimental results showed that the combined OA–TOP ligand was crucial for changingthe surface energy and controlling the particle shape over a broad size range. It wasfound that the particle sizes could be controlled by varying the molar ratios ofNa(CF3COO)/Re(CF3COO)3 (Re = Y, Yb, andEr). A high Na/Re ratio accelerated the cubic-phase transition and promoted the growth of smallerβ-UCNPs. Theformation of β-UCNPs was classified into kinetic and diffusion controlled stages, depending on the reactiontemperature and the dominant crystalline phases formed in each stage. In stage I, 250–310 °C, NaF generation wasthe limiting step and α-UCNPs were formed via a ‘burst of nucleation’. In stage II, above310 °C,the α-UCNPs formed were re-dissolved and the growth ofβ-UCNPs was a diffusion controlled process governed by the Gibbs–Thompsoneffect. A quasi-steady-state species assumption for NaF and a chemical potentialequilibrium in the solution were introduced to explain the particle size dependence onNa/Re ratios. The study of UC luminescence showed that the UC intensity was proportional to the sizes ofthe β-UCNPs.