The use of lead zirconate titanate ceramics for high power application must have superior mechanical properties, the composition Pb0.98Sr0.02(Mn1/3Sb2/3)(0.05)Zr0.48Ti0.47O3 + x wt% SiO2 (x = 0-1.0) have been studied for this purpose. The microstructures were observed to study the improvement of mechanical properties by SEM and TEM. The nano-scale secondary phase of SiO2 and PbSiO3 were observed on the grain boundary and the twinned ZrO2 was found surrounded by PMS-PZT perovskite phase. The mechanism to the occurrence of twinned ZrO2 and its content increased with increasing Si concentration are discussed in the paper. The occurrence of the nonferroelectric phase such as ZrO2, SiO2 and PbSiO3 on the grain boundary should be the main reason for great improvement of fracture strength..