In this paper is presented the structure and proprieties of the cast magnesium alloys as cast state and after heat treatment cooled with different cooling rate, depending on the cooling medium (furnace, water, air). For investigations samples in shape of 250x150x25 mm plates were used. The presented results concern X-ray qualitative and quantitative microanalysis as well as qualitative and quantitative X-ray diffraction method, tensile tests, hardness measurement. In the analysed alloys a structure of α %solid solution and fragile phase β (Mg17Al12) occurred mainly on grain borders as well as eutectic and phase AlMnFe, Mg2Si. Investigation are carried out for the reason of chemical composition influence and precipitation processes influence to the structure and mechanical properties of the magnesium cast alloys with different chemical composition in as cast alloys and after heat treatment.