We reported the preparation of cubic (α-) and hexagonal (β-) NaYF₄ particles in high boiling organic solvents 1-octadecene (ODE) and oleic acid (OA), through a thermal decomposition synthetic route. The as-prepared products were characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), photoluminescence (PL) spectra. By tuning the reaction temperature and time, we could manipulate the morphology, size, and crystal phase of the products. It was found that the α→β phase transition was favored by high reaction temperature and long reaction time. Highly uniform β-NaYF₄ hexagonal submicroplates were obtained from α-NaYF4 nanoparticles by prolonging the reaction time. The Yb³⁺-Tm³⁺ co-doped β-NaYF₄ hexagonal submicroplates showed s_{Subscript text}trong blue _{Subscript text}and weak red upconversion luminescence under 980nm laser diode excitation. In addition, the dependence of the UC emission intensities upon pump power was investigated.