K424 alloy was a kind of cast nickel-base superalloy that was rich in Al and Ti element and low density.Turbine blades made of this alloy cracked after overheated condition.In order to investigate the failure mechanism of K424 alloy blades，the morphology，composition and microstructure was analyzed by means of stereoscopic microscope，optical microscope，scanning electron microscope（SEM），energy dispersive spectroscopy（EDS）and micro-hardness testing machine.The analysis results show that the cracks in the blades have interdendritic morphology and the oxidation layer thickness on the fracture is more than 1 μm；the fracture mode is creep fatigue and high temperature oxidation.After overheated condition，the cubic γ’ phase（size~0.4 μm）transforms into secondary spherical γ’ phase（size ~0.08 μm），thus impairing the anti creep property.As a result，cracks initiate at dendrite grain boundaries，insular γ/γ′ eutectic-matrix interface and bulk carbide-matrix interface where stress concentration，deformation uncoordination and strength weaken position exists.After crack initiation，the Al，Ti and Cr elements in the fracture surface endure selective oxidation at an oxidizing atmosphere，resulting in an increasing volume and thus promote the expansion of cracks.