The WE43 cast magnesium alloy, containing yttrium and rare earth elements, remains stable at temperatures up to 300°C, according to the manufacturer, and therefore it is considered for a possible application in the aerospace and automotive. Usually, it is cast gravitationally into sand moulds and used for large-size castings that find application in the aerospace industry. After the casting process any possible defects that might appear in the casting are repaired with the application of welding techniques. These techniques also find application in renovation of the used cast elements and in the process of joining the cast parts into complex structures. An important factor determining the validity of the application of welding techniques for repairing or joining cast magnesium alloys is the structural stability and the stability of the properties of the joint in operating conditions. In the literature of the subject are information on the properties of the WE43 alloy or an impact of heat treatment on the structure and properties of the alloy, however, there is a lack of information concerning the welded joints produced from this alloy. This paper has been focused on the analysis the microstructure of the welded joints and their mechanical properties at elevated temperatures. To do this, tensile tests at temperatures ranging from 20°C to 300°C were performed. The tests showed, that up to the temperature of 150°C the crack occurred in the base material, whereas above this temperature level the rapture occurred within the weld. The loss of cohesion resulted from the nucleation of voids on grain boundaries and their formation into the main crack. The strength of the joints ranged from 150 MPa to 235 MPa, i.e. around 90 % of strength of the WE43 alloy after heat treatment (T6). Also performed a profilometric examination was to establish the shape of the fracture and to analyze how the temperature affected a contribution of phases in the process of cracking. It was found that the contribution of intermetallic phases in the process of cracking was three times lower for cracks located in the area of the weld.