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The creep properties, microstructural characteristics and creep mechanisms of as-cast Mg-5Bi-5Sn (BT55) alloy without and with Mn (BTM550) addition were investigated via creep at 423, 448, and 473 K as well as stresses of 30, 50 and 75 MPa. The results indicate that adding Mn can result in the formation of primary and the dynamic precipitated α-Mn phases. In addition, the morphology of the precipitated Mg3Bi2 phase and the orientation relationship between Mg2Sn precipitates and α-Mg can be effectively modified. Tailoring the microstructural characteristics is responsible for the improved creep performance of BTM550 alloy. The dominant creep mechanisms in BT55 and BTM550 alloys are dislocation cross-slip and climb, respectively. Furthermore, twinning and pyramidal 〈c + a〉 slip play an assisting part in both alloys during creep process.