ABSTRACT: The presented paper brings the result of Mg-alloy AZ 91D metallographic and fractographic fracture surfaces analysis, fatigue crack propagation and fatigue life measured at high-frequency cyclic loading. Microstructure of magnesium alloy AZ 91D specimens after thermal processing is created by uniform polyhedric grains of phase δ. At some places, within the grain boundaries there has been found the presence of small amount of discontinual precipitates consisting of fine lamellas of phase Y(electron compound Al12Mg17) in matrix of δ phase. Experimentally measured values of the fatigue resistance of alloy AZ 91D has been caused, first of all, by the presence of cast defects (micropipes, structural heterogeneities). The occurrence of these defects, their character, size, orientation, amount and location on the surface and in subsurface layers influenced in a negative way and to a various degree the strength the fatigue resistance of the studied alloy. The fatigue behavior and character of the fatigue fracture of the alloy AZ 91D are highly dependent on structural factors and on the level of technical perfection of their production, the character of the fracture surface morphology depended on the size of stress amplitude a to a considerable degree, due to strong structural heterogeneity and occurrence of large amount of cast defects (first of all, usually appearing microscopic cavities) it was not possible to show really the influence of the amplitude size of the applied strain on the initiation and micro-mechanism of fatigue failure development.