In periodic cyclic structures, small property irregularity of their substructured often causes significant difference in their dynamic responses, which results in unpredicted premature failures. The small irregularity and the resulting phenomenon are called the mistuning and the vibration localization, respectively. In this paper a simple coupled multi-pendulum system is employed to investigate the effects of the stiffness mistuning and the phase difference in excitation force on the vibration localization of periodic cyclic structures.