An investigation of the crack effect on the mode localization of nearly cyclic symmetry rotating bladed-disk is studied. A disk consisting of periodic shrouded blades is used to simulate the weakly coupled periodic structure. Blades on the system are modeled as cantilever beams which are approximated as Euler-Bernoulli beams. A crack blade on the system is regarded as a disorder of the system. That the mistuned system with weak coupling might occur the mode localization phenomenon, it is aimed to study in this paper. Galerkin method is used to derive the mode localization equations of the mistuned system with the consideration of crack effect. The effects of the location and depth of crack of the blade on the localization phenomenon of a rotating blade-disk system were investigated numerically. Numerical results indicate that the crack effect may affect the localization phenomenon and response.