Room temperature fatigue crack growth behaviour of un-exposed(as-received) and high temperature air-exposed(at 1100°C for 24 to 1000 h) CMSX-10 was investigated. A small compact tension specimen with  loading direction and  fatigue crack growth direction was used. The fatigue crack growth results were analyzed using a stress intensity parameter K . It is shown that the air-exposure has no deteriorative effect on the crack growth behaviour, even though the microstructure has changed by the exposure(coarsening of γ' ). The crystallographic facet fracture of (1-11) and (-111) planes appeared and the intersection of the planes forms sets of lines(ridge and valley) parallel to  direction. The fatigue crack tip front is schematically shown.