We report here on an in situ experimental analysis of the growth processes of tin dioxide nanoparticles (with mean particle radius of less than 5 nm) and the oriented attachment of these particles onto SnO2 nanobelts, used as single crystalline substrates, through electron irradiation by transmission electron microscopy. Characterization by high resolution transmission electron microscopy indicates that grain growth occurs in the solid state by a two-step process consisting of grain rotation followed by coalescence. The grain rotation is likely induced by the thermal energy provided by the electron beam. Coalescence or oriented attachment occurs preferentially on particles with similar crystallographic orientations.