Rotary bending fatigue tests were conducted in order to investigate the fatigue behaviour and fracture mechanisms of extruded Mg2Si-reinforced magnesium (Mg) alloys. Mg2Si particles were mixed with Mg alloy (AZ31) powder to form billets and then extruded at two different working temperatures of 685K and 646K, where the materials extruded at high and low temperatures were denoted as Mg2Si-H and Mg2Si-L, respectively. The grains of the matrix were finer in Mg2Si-L than in Mg2Si-H. The proof stress and tensile strength were independent of working temperature and increased compared with a conventional extruded AZ31 alloy, with a decreased in ductility. The fatigue strength of Mg2Si-H was almost the same as that of the extruded AZ31 alloy, but the fatigue limit was lower, while the fatigue strength of Mg2Si-L was higher than that of Mg2Si-H and the extruded AZ31 alloy. Both crack initiation and crack growth resistances in Mg2Si-L were improved, which could be attributed to high bonding strength of Mg2Si/Mg interfaces and fine grain refinement of the matrix.