Perovskite-type BaSnO
3
powders with a particle size of 30-60 nm have been prepared from tin oxide hydrate (SnO
2
∙
x
H
2
O) gel via the hydrothermal synthesis route. The reactivity of the synthesized SnO
2
∙
x
H
2
O was found to be dependent on the pH value of the mixture of tin chloride and the ammonia solution. The most reactive SnO
2
∙
x
H
2
O gel was obtained at a pH value near 2. The hydrothermal products derived from this reactive gel were characterized with thermograrimetric/differential thermal analysis, Fourier transform infrared spectrometry, X-ray diffraction and high-resolution scanning electron microscopy. The BaSn(OH)
6
phase in the as-prepared powder was found to convert into an amorphous phase other than into the BaSnO
3
phase. The BaSnO
3
powder crystallizing from the amorphous phase at 260
°C for 4 h has better sintering properties than the commercial one.