A study has been undertaken to reveal the creep properties of an experimental magnesium alloy, Mg-1.7RE-0.3Mn, and its microstructure after creep. Transmission electron microscopy examinations on the samples over aged during creep showed that extensive precipitation of Mg12RE type intermetallics took place on dislocations in the form of long single rows. Other precipitates of Mg12RE with lozenge morphology were observed to be distributed more uniformly inside the grains. Precipitates types have been identified via X-ray and electron diffraction, and microanalysis techniques. The relatively low creep rate compared to the commercial magnesium alloys was measured. A high value of activation energy, 240 kJ/mol, was calculated. The creep resistance and the activation energy of the alloy were discussed in terms of microstructural observations.