Abstract: Magnesium aluminophosphate (MAPO-5) molecular sieve was synthesized by hydrothermal method. The crystal structure of MAPO-5 has been studied by X-ray diffraction, electron probe, ³¹P-Nuclear Magnelic Resonance and infrared spectra. The parameters of the unite cell and chemical composition of MAPO-5 have been determined. Some physicochemical properties of MAPO-5 such as adsorption capacity, thermal stability and acid have been investigated. These results illustrates that Magnesium enter the framework of MAPO-5 molecular sieve.