This paper describes a general hydrothermal route for the synthesis of a series of rare-earth orthophosphates, LnPO4·xH2O (Ln=La, Y, Ce, Sm, Gd, Tb, Dy, Er, and Yb), with a mean diameter of ∼100 nm. The procedure involves the formation of homogeneous, transparent, metal–citrate–ethylenediamine tetraacetic acid (EDTA) gel precursors using both citric acid and EDTA as the complexing agents, followed by hydrothermal mineralization to yield the final LnPO4·xH2O nanocrystallites. Techniques of X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscope have been used to characterize the as-synthesized LnPO4·xH2O nanocrystals. Furthermore, photoluminescence (PL) characterization of the Eu3+-doped LnPO4·xH2O nanocrystals was carried out. The results indicate that the PL property changes considerably with the ionic radii of the rare-earth elements.