In this study, an analytical method is proposed to correlate local damage variables such as relative crack depth and crack tip opening displacement with a newly defined global damage index for a concrete beam under fatigue loading. This global damage index may be used to assess the response of a degraded concrete beam under service loading. The damage is assumed to appear in the form of a major crack that propagates under constant amplitude fatigue loading. The progressive cracking phenomenon is modeled within a finite element framework using a crack beam element, which takes into account the compliance variation due to discrete cracking within the member. The flexural stiffness degradation of the member is computed based on an Eigen analysis of the global stiffness matrix. It is seen that the degree of flexural stiffness degradation due to discrete cracking is the same for geometrically similar specimens when the relative crack depth is used as a local damage parameter. Further, in order to improve the accuracy of the response prediction using the above global damage index, another global damage parameter is defined based on the nature of applied loading.