The　microstructural　defects　of　nanocrystalline　SnO2　thin　films　prepared　by　pulsed　laser　deposition　have　been　investigated　using　transmission　electron　microscopy,　high-resolution　transmission　electron　microscopy　and　Raman　spectroscopy.　Defects　inside　nanocrystalline　SnO2　thin　films　could　be　significantly　reduced　by　in　situ　annealing　SnO2　thin　films　at　300　degrees　C　for　2　h.　High-resolution　transmission　electron　microscopy　showed　that　the　stacking　faults　and　twins　were　annihilated　upon　in　situ　annealing.　In　particular,　the　inside　of　the　SnO2　nanoparticles　demonstrated　perfect　lattices　free　of　defects　after　in　situ　annealing.　Raman　spectra　also　confirmed　that　the　in　situ　annealed　specimen　was　almost　defect-free.　By　using　in　situ　annealing,　defect-free　nanocrystalline　SnO2　thin　films　can　be　prepared　in　a　simple　and　practical　way,　which　holds　the　promise　for　applications　as　transparent　electrodes　and　solid-state　gas　sensors.　(C)　2009　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.