Na super ionic conductor (NASICON)-type LiTi2(PO4)(3) as a promising solid electrolyte has been widely used in all-solid-state lithium batteries. However, it still suffers from a low ionic conductivity. Herein, a hydrothermal-assisted solid-state reaction has been developed to synthesize rhombus-like Li1.3Al0.3Ti1.7(PO4)(3) (LATP) powder. Rhombus-like intermediate species of aluminum-doped NH4TiOPO4 and Li3PO4 are obtained by hydrothermal process, and subsequently the intermediate species are further preheated at 600 degrees C to become LATP phase without changing the rhombus-like morphology. The as-prepared LATP powder with high crystallinity, rhombus-like morphology and uniform size distribution is beneficial to improving the sintering density of LATP ceramic solid electrolyte. Furthermore, influences of preheating treatment temperature and final sintering temperature on the ionic conductivity of LATP solid electrolyte are also discussed. The ionic conductivity of LATP solid electrolyte measured at 303 K with 97.3% relative density is as high as 0.48 mS cm(-1). Additionally, low active energy (0.30 eV) and low electronic conductivity (similar to 10-6 mS cm(-1)) are obtained. The synthesized method in the present work provides a new thought of design for other solid electrolytes with outstanding electrochemical performance.