Barium titanate (BaTiO3, BT) potassium niobate (KNbO3, KN) solid solution system (05BT-0.5KN) ceramics with various microstructures were prepared by two-step sintering method, and their piezoelectric properties were investigated. For 0.5BT-0.5KN ceramics, two phases, ferroelectric tetragonal and ferroelectric orthorhombic, coexisted in different grains at room temperature, owing to the limited solid solution system. The volume fraction of interface region between BT-rich tetragonal and KN-rich orthorhombic grains was controlled by sintering temperatures, and increased with decreasing sintering temperatures. Apparent piezoelectric constant d(33)* was measured using slope of strain vs. electric field curves. As the results, the d(33)* increased with decreasing sintering temperatures, which revealed that interface region between tetragonal and orthorhombic grains could contribute to enhancement of piezoelectric properties.