The crystal structure and piezoelectric properties of ternary ceramics x(Bi1/2Na1/2)TiO3–y(Bi1/2K1/2)TiO3–zKNbO3 (abbreviated as BNT-BKT-KN100x/100y/100z) system have been investigated. Effects of BKT and KN on the crystalline structure were examined. The morphotropic phase boundary (MPB) phase diagram of BNT-BKT-KN ternary system was determined by x-ray diffraction patterns from sintered specimens. Near the MPB, BNT-BKT-KN82/16/2 ceramics had good performances with piezoelectric constant d33=215 pC/N which reached a maximum value in this ternary system and planar electromechanical coupling factor kp=35%. Besides, BNT-BKT-KN87/10/3 ceramics had good performances with piezoelectric constant d33=138 pC/N and electromechanical coupling factor kp=38% which attained a maximum value in this ternary system.