Design of the polymorphic phase composition in the (0.975-y)BaTiO3-0.025SrTiO(3)-yBaSnO(3); BT-ST-yBSn ternary system was based on the ferroelectric phase diagram. The dense ceramic of BT-ST-yBSn, with y = 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10 compositions, was fabricated successfully via the solid-state reaction method. The effect of Sn substitution on the ferroelectric phase transition and piezoelectric properties was explored in order to achieve high-performance piezoelectric properties. All of the ceramics exhibited pure perovskite structures. Orthorhombic to tetragonal phase transition was evidenced clearly as a function of Sn content. The orthorhombic to tetragonal phase transition shifted close to ambient temperature by increasing the Sn content. The coexistent tetragonal and orthorhombic phases were exhibited at the composition, y = 0.04, and showed outstanding dielectric and piezoelectric properties, maximum relative permittivity (epsilon (r max)) of 11500 and piezoelectric coefficient (d (33)) of 450 pC/N. An outstanding reversible strain of about 0.12%, with a normalized piezoelectric coefficient (S (max) /E (max)) of 1280 pm/V at a low electric field (10 kV/cm), was observed clearly at the composition of the coexistent phase. The BT-ST-BSn ceramics are the most promising candidate for lead-free piezoelectric materials.