Tensile creep behaviour of the die-cast magnesium alloys AZ91 and AE42 has been studied at temperatures between 85oC and 200oC and at stresses in the range from 30 to 100 MPa. Microstructural investigations, mainly by TEM, have been performed on the selected crept samples to characterise the microstructural change during the plastic deformation, which reveals several phenomena related to the creep process including formation of dislocation sources, denuded zones around grain boundaries and microvoids, and changes in the nature of intermetallic phases. The active creep mechanisms have been discussed on the basis of the creep data in combination with the microstructural change during creep.