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The fatigue behavior of as-cast Mg-12%Zn-1.2%Y-0.4%Zr alloy has been investigated. The S-N curve showed that the fatigue strength at 107 cycles was 45 MPa. Scanning electron microscopy observations on the surfaces of the failed and unfailed specimens (after up to 1 X 10(7) cycles) suggested that the slip bands could act as preferential sites for non-propagating fatigue crack initiation, and the I-phase could effectively retard fatigue crack propagation (FCP). The macro fracture morphology clearly indicated that the overall fracture surface was composed of three regions, i.e. a fatigue crack initiation region (Region 1), a steady crack propagation region (Region 2) and a tearing region (Region 3). High-magnification fractographs showed that only porosities can act as the crack initiation sites for all specimens. Moreover, for specimens with fatigue lifetimes lower than 2 x 10(5) cycles, the cracks mostly initiated at the subsurface or surface of the specimen. However, when the fatigue lifetime was equal to or higher than 2 x 10(5) cycles, the fatigue crack initiation sites transferred to the interior of the specimen. The maximum stress intensity factors corresponding to the transition sites between Regions 1, 2 and 3 were 2 and 4.2 MPa m(1/2), respectively. When the maximum stress intensity factor K(max) was lower than 4.2 MPa m(1/2), in the steady crack propagation region, due to the retarding effect of I-phase/alpha-Mg matrix interfaces, the fatigue cracks tended to pass the I-phase/alpha-Mg matrix eutectic pockets directly and propagated through the grain cells, resulting in the formation of many flat facets on the fracture surface. However, when the maximum stress intensity factor was higher than 4.2 MPa m(1/2), in the sudden failure region, the rigid bonding of I-phase/alpha-Mg matrix interfaces was destroyed and the cracks preferentially propagated along the interfaces, which resulted in the fracture surface being almost completely composed of cracked I-phase/alpha-Mg matrix eutectic pockets. Based on microstructural observation and the fracture characteristics of the two regions, it is suggested that with an increase in crack tip driving force, the FCP mode changes from transgranular propagation to intergranular propagation. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.