As a wide band-gap semiconductor，SnO2 films have attracted much attention due to their novel optical and electronic properties. It has been reported that the physical properties can be quite different when the size of SnO2 is reduced to nanometer scale due to the large surface-to-volume ratio and the quantum size effects，which may be applied in many kinds of devices, such as solar cells sensors. etc. It is interesting to study the synthesis of SnO2 nanoparticles and their physics properties. In the present work，a soft chemical technique was used to prepare SnO2 nanoparticles with uniform size and good crystallization in alkalescent solution. The surfactant was added during the preparation process to control the growth and agglomeration of crystal precipitates in the solution. X-ray diffraction spectra and transmission electron microscopy were used to characterize the structures of SnO2 nanoparticles before and after thermal annealing. It was found that the nanocrystalline SnO2 particles can be formed by the present technique and the size is about 4 nm with good crystallinity. With changing the preparation parameters，the size of nanocrystalline SnO2 particles is changed. Post thermal annealing at various temperatures （400—1000 ℃） can promote the crystallization and the size of formed particles was increased with increasing annealing temperature. Optical absorption spectra were used to see the change of the optical properties for samples prepared under different conditions. It was found that the optical band gap is enlarged in nanocrystalline SnO2 particles compared with its bulk counterpart, which can be attributed to the quantum confinement effect. The red-shift of the optical band gap with the particle size supported the quantum size effect. A broad photoluminescence band in the range of 350—750 nm can be detected in the annealed samples and the intensity was significantly enhanced after the thermal annealing. The luminescence peak energy was kept at 390 nm which was independent of the particle size. This luminescence band can be ascribed to the luminescence center associated with the oxygen vacancies on the SnO2 particle surface.