A BaSnO 3 powder with a crystallite size of 27.6 nm has been prepared through a hydrothermal reaction of a peptised SnO 2 .xH 2 O and Ba(OH) 2 at 250 o C and the following crystallization of this hydrothermal product at 330 o C. The peptisation of the SnO 2 .xH 2 O gel is dependent on the pH value. Through peptisation the mean particle size of SnO 2 .xH 2 O in the aqueous solution has been decreased by a factor of 100 to 8nm. A limited agglomeration in the sol-prepared powder has been observed under the microscope. The structure evolution and crystallisation behaviours of the sol-prepared powders were investigated by TG-DTA, IR and XRD. The BaSn(OH) 6 phase in the as-prepared powder transforms into an amorphous phase at 260 o C, from which the BaSnO 3 particles nucleate and grow with an increase in temperature. The single-phase BaSnO 3 powder has been obtained at a temperature as low as 330 o C. This sol-prepared powder is more sinter-reactive than the gel-prepared powder and can be sintered to a ceramic with 90.7% of the theoretic density.