Abstract: The dynamic dissipation behaviors of boundaries of polar nanoregions (PNRs) in PMN-15PT ceramics were investigated via internal friction and modulus measurements. An internal friction peak depended strongly on the variation rate of temperature θ and applied vibrating frequency f was observed in the temperature range of relaxor ferroelectrics. The nature of the internal friction peak was suggested to be associated with the dynamic process of the motion of PNRs’ boundaries driven by the variation of temperature and the feature of PNRs’ boundaries could be described by the dissipation function model proposed for the phase interface dynamics of firstorder phase transition. By comparison to other kinds of ceramic samples with normal sized phase interface, it was found that the interface of PNRs is a class of soft interface with small dynamic restoring coefficient and weak interaction between the interfaces and applied driven field.