IR-to-visible up-conversion fluorescent nanocrystals of hexagonal-phase NaYF4 20% Yb,2%Er and NaYF4 20%Yb,2%Tm have been synthesized by decomposition of multiprecursors of CF3COONa, (CF3COO)(3)Y, (CF3COO)(3)Yb, and (CF3COO)(3)Er/(CF3COO)(3)Tm in oleylamine at 330 degrees C. The average particle size is 10.5 +/- 0.7 nm (from random measurements of 200 particles from five transmission electron microscopy images) and 11.1 +/- 1.3 nm (from dynamic-light-scattering measurements). The upconversion fluorescence intensity of the hexagonal nanocrystals in this work is much higher than that of other cubic-phase NaYF4:Er nanocrystals including the ones in this work (by a factor of 7.5). Mechanisms for nucleation and growth of the hexagonal-phase nanoparticles are proposed. These nanocrystals are easily dispersed in organic solvents, producing a transparent colloidal solution. The hydrophobic surfaces of the particles are made hydrophilic using a bipolar surfactant. These nanoparticles and their dispersions in various media have potential application sin optical nanodevices and bioprobes.