Nowadays, metal foams have attracted much attention because of their unusual combination of physical and mechanical properties. There are various methods to produce these structures. The Gasar method is much better than the traditional techniques in terms of effective porosity control, pore size and orientation. The particular condition of this method and its expensive production process are the limitations which the material engineers confront to them. For these reasons, recently, several researches have been developed a new fabrication technique using a thermal decomposition method in order to produce foams characterized by Gasar structures which made of different metals. In this study, according to magnesium potential applications in different industries, fabrication the lotus- type porous magnesium at atmospheric pressure using the thermal decomposition of two types of polymers called Viscous Rayon and Polypropylene with C 6 H 10 O 5 and C 3 H 6 molecular formulas, respectively, examined. In this method, the Hydrogen which is produced by thermal decomposition of polymer foam -causing agent, dissolved in Magnesium and formation of pores. The CELEMEX software was applied to determine the effect of foam causing factor on the structure of foam including amount of porosity and pore diameter. X- ray diffraction, thermal analysis and infrared spectroscopy are used to analysis the gas which achieved from decomposition of factors causing foam in argon atmosphere. This is the way of identify foam causing agent according to productions due to decomposition of that with temperature increasing. With increasing amount of foam-causing agent, the porosity increases to a maximum, but then did not change because of the solubility of hydrogen from a certain amount is constant. The average pore diameter is almost constant and not varied so much. Respectively, the highest porosity created through Viscous Rayon and Polypropylene fibers has been about 52% and 23%. The maximum porosity achieved from the Viscous Rayon fibers because of formation of a viscous slag on the surface of molten. Formation of slag increased the partial pressure of the gas in the molten metal. Consequently, the gas solubility increase, thus, it will be possible to build structures with higher porosity. But in manufacturing process using Polypropylene fibers, due to the lack of slag formation on the surface molten, there are not any factors to prevent gas escaping and the amount of porosity will be decreased. Therefore, it has been concluded that the foam manufacturing process through Viscous Rayon fibers provides extra porosity compared with the foam manufacturing process using Polypropylene fibers. Keywords: Magnesium foam, Gasar method, Thermal decomposition, Viscous Rayon fiber, Polypropylene fiber, porosity.