Nanocrystalline Ln(3+)-doped YF3 phosphors have been synthesized via a facile sonochemistry-assisted hydrothermal route. YF3 nanoparticles are demonstrated to be a good host material for different lanthanides. Varying the dopants leads to different optic, cal properties. In particular, the feasibility of inducing red, green, and especially blue emission in the Yb3+/Er3+ co-doped YF3 sample by up-conversion excitation in the near-infrared region is demonstrated. Such unusually strong 411 nm blue up-conversion emission has seldom been reported in other Yb3+/Er3+-doped systems. The up-conversion mechanisms have been analyzed.