Stable phase alpha-manganese molybdate (alpha-MnMoO4) rods, wolframite structure MnMoO4 hollow olive-like spheres and hexahedron-like manganese molybdate monohydrate (MnMoO4 center dot H2O) were selectively synthesized by a simple hydrothermal method at 150-200 degrees C. Experiments showed that the reaction temperature, pH, molybdenum source and ligand played very important roles in controlling the polymorph and morphologies of the product. The samples were characterized by X-ray powder diffraction and scanning electron microscopy. Possible formation process of the product was proposed.