Nanocrystalline BaSnO
3
with a primary particle size of 40-60 nm was prepared through hydrolysis of a barium tin isopropoxide and following crystallization. The thermal decomposition, the crystallization and the microstructure of the obtained powders were investigated with the help of TG-DTA, IR, XRD, HRSEM and HRTEM. The organic rest groups in the as-prepared powder decompose thermally at 350 °C, which is accompanied by the building of BaCO
3
that disappear again at 600 °C. The crystallization of BaSnO
3
takes place at 500-600 °C. Single-phase BaSnO
3
powders have been obtained at a temperature as low as 600 °C. The amorphous as-prepared powder shows a cluster structure. Nucleation of BaSnO
3
beginning at 350 °C was observed under HRTEM, and the spherical nano-particles of BaSnO
3
calcined at 760 °C crystallize well and are strongly aggregated. The presented results indicate a heterogeneous nucleation and growth mechanism by the formation of BaSnO
3
.