Monodisperse water-soluble LaF(3):Ln(3+) nanocrystals (NCs) have been successfully fabricated via a fast, facile and environmentally-friendly microwave-assisted modified polyol process with polyvinylpyrrolidone (PVP) as an amphiphilic surfactant. The obtained NCs can be well dispersed in hydrophilic solutions with small sizes in the range of 9-12 nm. The LaF(3):Ln(3+) NCs (Ln = Eu, Nd, Ce, Tb, Yb, Er, Yb, Ho and Yb, Tm) have the unique feature of up-down conversion from visible to NIR emission owing to the ladder-like arranged energy levels of Ln(3+) and in particular, the high efficiency upconversion of the two-photon, obtained from excitation by a continuous 980 nm laser. This investigation focuses on both the up and down conversion fluorescence properties of water-soluble monodisperse crystalline LaF(3):Ln(3+) NCs in such a small size. Furthermore, the three-dimensional PDMS rod-like fluorescence displays and a silica surface modification by a core/shell structure on the obtained NCs can improve the biocompatibility, indicating potential applications in optical 3D devices and as bio-probes.