LaF3 : Yb3+, Er3+ nanoparticles were synthesized through a simple hydrothermal method. The nanoparticles were well-crystallized and exhibited hexagonal structure with fine morphology, as indicated by powder X-ray diffraction, electron diffraction, and transmission electron microscopy. The nanocrystals present a bright-green upconversion luminescence under the 978-nm excitation of a laser diode, which provides a promising upconversion phosphor for optoelectronic or biological applications.