The polarization relaxation phenomenon in a 40-nm-thick epitaxial BiFeO3 thin film grown on a ( 001) SrTiO3 substrate with SrRuO3 bottom electrode, was studied in nanoscale using dual-frequency resonance-tracking piezoresponse force microscopy. The as-grown film shows highly irregular mosaic domain pattern. The nucleation of reversed domains followed by domain wall propagation and domain coalesce was observed during relaxations. The polarization relaxation follows a stretched exponential model f = 1-e(-k(t-t0)n) with parameters t(0)=2894 s, n=0.50 and k=6.04e-4. Local polar defects act as nucleation centers and the time-dependent depolarization field is the driving force for polarization relaxation. Crown Copyright (C) 2010 Published by Elsevier B. V. All rights reserved.