The microstructure and creep behavior of Mg/Al composite crankcases cast with three alloy formulations of the Mg-Al-Sr alloy AJ62 have been investigated. Overall 12 components were used within this study. Multi-level creep tests were conducted to evaluate the creep properties at stresses up to 90 MPa and temperatures up to 473 K. Microstructure observations including phase characterization and in-depth dislocation analyses were performed in the as cast condition and after creep testing. The tensile creep testing revealed a distinct primary creep and a high stress exponent up to a value of 10. The threshold stress concept was applied, which yields to an effective stress exponent of 5 indicating a strengthening effect due to particle-dislocation interaction. Transmission election microscopy (TEM) of the microstructure revealed the continuous precipitation of -Mg 17 Al 12 in the ␣-Mg matrix near the interdendritic regions during creep. In addition, a fine-dispersed nano-scaled Al-Mn phase, probably Al 8 Mn 5 , was observed in the ␣-Mg matrix in all samples under investigation. According to an in-depth TEM analysis of the dislocation structure, slip of non-basal a dislocations and c + a dislocations is activated in addition to basal slip even at 423 K and very low stress (15 MPa). Furthermore, the TEM images reveal a strong interaction between dislocations and the Mg-Al and Al-Mn matrix precipitates. Hence, matrix strengthening by well-distributed precipitates could be one factor for the excellent creep resistance of AJ-alloys. Despite of the matrix precipitates, the substitution of the eutectic phase Al 4 Sr by Mg 9 Al 3 Sr in one of the alloys seems to be the major difference in the investigated alloys and should therefore account for the differences in creep rate and creep strain.