Abstract Magnesium alloys has been used for a wide variety of applications, namely from the reason of their low density and high strength–to–weight ratio. Their material selection is preceded by analysis of many factors, including considerations concerning mechanical properties, design, environmental, urbanization, recycling, cost, availability etc. Low inertia, which results from its low density, is advantageous in rapidly moving parts, for example automobile wheels and other automobile parts. The similar situation is in the aeronautical market and air–frame application. Scope of utilisation of foundry magnesium alloys is continuously being extended, so if we want to operate as competitive producers, it is necessary to investigate very actively properties of individual alloys, optimise their chemical composition, study issues of their metallurgical preparation, verify experimentally their casting properties and conditions of successful casting of castings by individual methods, including heat treatment. Recently, however, increases also utilisation of formed magnesium alloys. Knowledge of plastic properties is very important for complex evaluation of magnesium alloys for commercial using. The paper presents the influence of changing testing conditions on plasticity of magnesium alloys with graduated aluminium content at cast state and after heat treatment. Alloys AZ91, AZ61 and AZ31 (after ASTM Standard) were used. Objective of the work was determination mechanical properties of magnesium alloys in dependence on method of its processing, including investigation of structure and fracture characteristics with use light metallographic analyse and SEM. Impact of previous deformation after equal channel angular pressing by ECAP method on mechanical properties of selected alloys was considered as another important factor. At special conditions an effect of superplasticity was observed.