The high temperature fatigue crack growth behaviour of the nickel base superalloys Alloy 718 and Rene 95 (specimen thickness = 4.1 mm) were investigated and compared with each other. Fatigue crack propagation (FCP) tests were carried out in laboratory air at room temperature and 600 degrees C by using C-T (compact tension) type specimen that were fatigue precracked at room temperature. Alloy 718 was found to provide the higher resistance to crack propagation under the present testing conditions. At 600 degrees C, in Alloy 718, the fracture path was of mixed type at low a nd transgranular at high Delta K(stress intensity factor range) values, while it remained intergranular in Rene 95 throughout the whole Delta K range tested. The difference in the crack growth rates of Alloy 718 with different thicknesses (4.1 mm and 13.0 mm) was related to their different fracture modes. The striation spacings, both at room temperature and 600 degrees C, of Alloy 718 were found to be proportional to the empirical equation proposed by Bates and Clark [2] but with a constant of 9.5 instead of 6. However, although the correlation between the microscopic FCP rate obtained from fatigue striation measurements - and hence the empirical equation - and the macroscopic FCP rate was pretty good at room temperature, it was found to be poor at 600 degrees C, indicating that, at 600 degrees C, striation formation alone did not control the fatigue resistance of Alloy 718 which is thought to account for the insufficiency of the COD (crack opening displacement) approach to correctly correlate the macroscopic FCP rates of Alloy 718 at these two test temperatures.