LaPO4:Eu3+ nanoparticles were prepared by the hydrothermal synthesis method at different temperatures. The phase composition and microstructure of nanoparticles were characterized by XRD (X-ray diffraction) and SEM (Scanning electron microscopy), respectively. The results reveal that the phase composition of the powders is dependent on the water bath temperature but the microstructure has little effect. The doped powders can be well crystallized at 120°C with the hexagonal monazite-type structure of the LaPO4 phase. The monoclinic structure of LaPO4 phase generated as the temperature increases. The sample has pure monoclinic structure of LaPO4 phase when the temperature increased to 200°C. It has been shown that all the nanocrystals obtained at different temperatures present a long rod-like shape. The analysis of the fluorescence spectra shows that the luminous intensity of the monoclinic structure of LaPO4:Eu3+ is stronger than the hexagonal structure.