Abstract: This paper presents calculations of the mode I stress intensity factors (K_I) for an auto-reinforced thick walled cylinder with one or two internal axial semi-elliptical surface cracks under two degrees of auto-reinforcement. The stress intensity factor is decomposed into two parts: K_IP and K_IR. K_IP is produced by the internal work pressure and K_IR is produced by the residual stress. K_IP and K_IR are calculated by 3-D finite element method from nodal point displacements at the crack surface. Besides, the superposition principle is applied in the determination ofK_IR. The general purpose finite element program SAP5,to which a few modification have been made, is used for the analyses. The results show that the existence of residual stresses can reduce the stress intensity factor K_I of internal crack efficiently. The higher the reinforced degree, the more obvious is this effect.