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In the gigacycle fatigue regime (106 to 109 Cycles), the micro-mechanism of fatigue crack propagation for a forged Mg-Zn-Y-Zr alloy subjected to temper treatment (T5) has been studied. High-magnified images of the different crack propagation regions indicate that when the effective stress intensity factor K(eff) is below 3.2 MPa m(1/2), in Region 1, the fracture surface is smooth covered with many flat facets. However, when the stress intensity factor is higher than 3.2 MPa m(1/2), in Region 2, many larnellar semi-cleavage planes caused by rod-like beta(1)', precipitates can be observed, resulting in a rather rough fracture surface. However, in Region 3, no lamellar semi-cleavage planes but lots of zonal distributed Mg(3)Zn(3)Y(2) phases can be observed. It suggests that with the increase of the crack-tip driving force, the preferential path for fatigue crack propagation changes from the rod-like beta(1)' precipitates to the zonal distributed Mg(3)Zn(3)Y(2) phases. In addition, two kinds of fatigue striations, i.e. striations formed on the flat fracture surface and striations formed on the cleavage steps, have been observed. Based on double-slip and plastic obtuseness at the crack tip, the forming mechanism of the fatigue striations on the cleavage steps has been discussed. (C) 2006 Elsevier B.V. All rights reserved.