Fine, yellowish powder of Pb2SnO4 with elongated crystalline morphology was synthesized hydrothermally in an alkaline system. To prepare a pure product, the mineralizer NaOH should be at a suitable concentration. In comparison with the solid-slate method, the hydrothermal synthesis of Pb2SnO4 requires much milder reaction conditions. For some metastable phases such as PbO, 3PbO-H2O and PbSnO3-2H(2)O transformed into Pb2SnO4 with increase in hydrothermal reaction temperature and reaction time. Pb2SnO4 is considered to be the thermodynamically stable phase in the alkaline hydrothermal system of PbO and SnO2. Reaction temperature and reaction time influenced the size and aspect ratio of the elongated particles. In this work, no PbSnO3 phase was found under any hydrothermal conditions, even though the molar ratio of the starting material was 1:1.