Highly crystalline MgSn(OH)(6) cubic and polyhedral nanoparticles were synthesized via hydrothermal method by adjusting the ethanol/water ratio. Mg2SnO4/SnO2 nanoparticles have been successfully obtained by subsequent calcining, which maintain the original morphologies of the precursor with shrank size and rough surface. The structural properties of Mg2SnO4/SnO2 heterostructures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis absorption spectroscopy, and N-2 adsorption/desorption isotherms. The photocatalytic activity of the Mg2SnO4/SnO2 nanoparticles was further evaluated by the degradation of methylene blue (MB) under UV illumination. The obtained results revealed that Mg2SnO4/SnO2 polyhedral nanoparticles have excellent performance on photocatalytic degradation of MB. It is assumed that the red shift of band gap, oxygen vacancies, and higher specific surface areas of polyhedral Mg2SnO4/SnO2 nanocomposites contribute to the enhancement in the photocatalytic activity.