A BaSnO
3
powder with a crystallite size of 27.6 nm has been prepared through a hydrothermal reaction of a peptised SnO
2
∙
x
H
2
O and Ba(OH)
2
at 250°C and the following crystallization of this hydrothermal product at 330°C. The peptisation of the SnO
2
∙
x
H
2
O gel is dependent on the pH value. Through peptisation the mean particle size of SnO
2
∙
x
H
2
O in the aqueous solution has been decreased by a factor of 100 to 8 nm. 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°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°C. This sol-prepared powder is more sinter-reactive than the get-prepared powder and can be sintered to a ceramic with 90.7% of the theoretic density.