Beijing Univ Posts & Telecommun, Beijing 100876, Peoples R China
; Acad Sinica, Inst High Energy Phys, Beijing 100049, Peoples R China
We define four Wave function Renormalization Constants (WRCs) for unstable particles under the LSZ reduction formula. By CPT conservation law we obtain a new wave function renormalization condition and determine the four WRCS. By calculating the gauge dependence of the physical amplitudes we demonstrate the consistency of the current wave function renormalization prescription with the gauge theory in standard model. We also prove that the conventional wave function renormalization prescription leads to physical amplitudes of gauge dependent.