High-temperature piezoelectric ceramics (0.95 - x)BiScO3-xPbTiO(3)-0.05Pb(Yb1/2Nb1/2)O-3 (BS-xPT-PYN, x = 0.56-0.64) were prepared by a conventional solid-state reaction method. X-ray diffraction and Raman spectra demonstrated that the ceramics convert from monoclinic phase (x <= 0.58) to tetragonal phase (0.62 <= x <= 0.64). A phase coexistence of monoclinic and tetragonal in the vicinity of the MPB (x = 0.60) enhances the ferroelectric polarizability by a dynamical conversion between two energy-degenerate states with d(33) = 392 pC/N, k(p) = 53.3% and P-r = 38.8 mu C/cm(2) and the operation temperature up to similar to 400 degrees C. Relaxor behaviors, which following the V-F law, are presented in this system and their dipole activation energy decrease for higher component x. The phase fraction versus temperature of the MPB (x = 0.60) was investigated by an in situ XRD, which gives an insight into the origin of the high-temperature piezoelectricity.