Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO 2 thin films onto glass substrates. The nanocrystallites and microstructures in SnO 2 thin films grown by PLD techniques have been investigated in detail by using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The PLD process was carried out at room temperature under a working pressure of about 2×10 −6 mbar. Experimental results indicate that thin films are composed of a polycrystalline SnO 2 and an amorphous SnO phase. In particular, the presence of such an amorphous SnO phase in the thin films greatly limits their practical use as gas-sensing devices. HRTEM observations revealed that SnO 2 nanocrystallites with tetragonal rutile structure embed in an amorphous SnO matrix, which are approximatively equiaxed. These approximatively equiaxed SnO 2 nanocrystallites contain a high density of defects, such as twin boundaries and edge dislocations. The grain growth of SnO 2 thin films may be discussed in terms of the coalescent particle growth mechanism.