The autofrettage process is normally used to produce favorable residual compressive stress field near the bore of a thick-walled tube, thus increasing its loading capacity and/or reducing its weight. This process retards the initiation and propagation of radial cracks in the bore N. On the other hand, the plastic deformation and hardening which occur in the cylinder due to autofrettage are likely to decrease the fracture toughness of the material, which, in turn, decreases the fatigue life. In the present work, the effect of autofrettage on fracture toughness is first investigated using standard arc-shaped specimens cut from tubes with different overstrains. It is found that increasing autofrettage overstrain to about 100% reduces the fracture toughness of the material by about 20%. Secondly, the effect of autofrettage on fatigue crack growth of internal cracks is studied experimentally using ring specimens. Furthermore, a mathematical model using the superposition principle is developed and used to predict fatigue crack growth for the speaimens used. Comparison between experimental and predicted results is performed and good agreement is generally obtained.It seems that there is an optimum value of autofrettage overstrain, which increases fatigue life and resistance to sudden failure. This value depends on the tube material and loading conditions. Further experimentation is yet to be done to confirm these results.