A quantitative relation between the morphotropic phase boundary (MPB) composition and the tolerance factor (t) in (Bi0.5Na 0.5)TiO3 (BNT)-based piezoelectric ceramics was established. The t value of the MPB compositions in BNT-based ceramics are around 0.990 ̃ 0.993 and is independent of the types of added compounds. In order to experimentally demonstrate it, two piezoelectric ceramic systems (l-x)(Bi0.5Na0.5)TiO3-x(Bal-aSr a)TiO3, a = 0.05 and 0.3 (BNBST5-X and BNBST30-X, x < 12%), were used. The X-ray diffraction patterns and dielectric properties measurements indicated that the MPB compositions were BNBST5-6 and BNBST30-8 and their corresponding t value were 0.9900 and 0.9903, respectively. The results confirm the relation between the MPB composition and t value and provide a method for designing new piezoelectric materials.