Abstract: Investigation of fatigue crack initiation and propagation has been carried out on smooth plate specimens of a nickel base cast superalloy treated by HIP. SEM observations showed that the crack initiated at carbides, microporosities and in the matrix near the microporosities. In the observations emphasis was put on the growth of cracks from 20μm to 2000μm long and the characteristics of micro/macro crack transition.A discontinuous and irregular crack growth rate of the short cracks （<1000μm）found in L-N plot indicates a marked influence of microstruc-ture, especially at grain boundaries. Although the transient growth rate of microcrack is quite complicated and sensitive to microstructure, the perturbation may be concealed by an average growth rate. The propagation rate of small crack is at least an order of magnitude faster than that of LEFM large crack.