Ferroelectric and piezoelectric properties of SrTiO3–modified (0, 3, and 5 mol%) 0.8Bi1/2Na1/2TiO3–0.2Bi1/2K1/2TiO3 lead-free piezoceramics were investigated as a model system in an attempt to lay a guideline for developing lead-free piezoelectric materials with large strains. Two guidelines, one for the choice of base composition and the other for the choice of chemical modifiers, were assumed from our current understanding of the mechanism involved. Dielectric permittivity of both poled and unpoled samples was measured and compared, leading to a conclusion that the frequency-independent anomaly (TF-R) is the temperature at which induced-ferroelectric order converts back to relaxor state. The correlation between TF-R and depolarization temperature (Td) was shown by the comparison with Td determined by thermally stimulated depolarization current measurements, whereas the ferroelectric-relaxor transition temperature TF-R was determined using poled samples. A large unipolar strain of 0.36% (Smax/Emax = 600 pm/V) at a driving field of 6 kV/mm was obtained at room temperature for a SrTiO3 content of 5 mol%. Temperature-dependent measurements of both polarization and strain from room temperature to 200°C revealed that the origin of the large strain is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation.