The synthesis of tin oxide (SnO2) nanoparticles by mechanochemical reaction (SnCl2 +Na2CO3) with NaCl as diluent and subsequent thermal treatment was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). Heat treatment of the as-milled powder at 600℃ in air and removal of NaCl through washing produced the SnO2 nanocrystallites with an average crystal size of about 28 nm, varying with the thermal treatment temperature. The mechanism of nanocrystallites growth is discussed.