High cycle fatigue behavior of MB8 magnesium alloy were investigated using an up-and-down load method. High cycle fatigue tests were carried out up to 107cycles at a stress ratio R=0.1 and frequency of 90Hz on specimens using a high frequency fatigue machine. Fatigue fracture surfaces of specimens that in the high cycle fatigue tests were also observed using a scanning electron microscope for revealing the micro-mechanisms of fatigue crack initiation and propagation. The results showed that fatigue limit of MB8 alloy at room temperature is 90.2 MPa under the numbers of cycle to failure Nf=107 conditions using up-and-down method calculation. The fatigue strength of the alloy is about 34% of its tensile strength. The micro-fatigue fracture surface of MB8 alloy included three representative regions. These regions are fatigue initiation area, fatigue crack propagation area and fatigue fracture area. Fatigue cracks of MB8 alloy initiate principally at surface and subsurface, and propagate along the grain boundary. The fatigue striations of fatigue crack propagation area are not clear. The fatigue fracture of test specimens show the rupture characteristics of dimple.