Well-dispersed crystalline tin dioxide (SnO2) nanoparticles were synthesized by a novel and simple water-in-oil (w/o) microemulsion-assisted hydrothermal process, using low-cost tin chloride as the starting material. The typical quaternary microemulsions of cetyltrimethylammonium bromide (CTAB)/n-pentanol/n-hexane/water were used as space-confined microreactors for the nucleation, growth, and crystallization of SnO2 nanoparticles under hydrothermal conditions. The techniques of XRD, TEM, HRTEM, SAED, EDS, FTIR, XPS, and N2 adsorption measurement were used to characterize the compositions and structures of obtained samples. The results show that the SnO2 nanoparticles have high specific areas (107-169 m2 g(-1)), small particle sizes (ca. 3.0 nm), high crystallinity, and narrow size distributions. The well-dispersed, uniform, and well-crystallized powders with microporous texture are favorable for gas-sensing applications. The as-developed microemulsion-assisted in situ crystallizing process can be extended to prepare other oxide, metal, and sulfide nanoparticles.