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 sSubscript texttrong blue Subscript textand weak red upconversion luminescence under 980nm laser diode excitation. In addition, the dependence of the UC emission intensities upon pump power was investigated.