X-ray powder diffraction has been used to investigate the cubic-tetragonal phase transformation and lattice constant in a BaSnxTi1-xO3 system (x = 0.0-0.2) at room temperature. The existing regions of tetragonal phase were strongly influenced by the amounts of tin (from x = 0.0-0.1). The mass fraction of the tetragonal phase decreases as substitution of tin for titanium increases until x = 0.1, while the mass fraction of the cubic phase increases simultaneously. The lattice constant of cubic phase and the cube roots of the unit-cell volumes of tetragonal phases indicate linear relations with the amounts of tin.